Multi-sensor mapping omnidirectional sonde and line locator

ABSTRACT

Portable locators are disclosed for finding and mapping buried objects such as utilities. A articulatable antenna node configuration and the use of Doppler radar and GPS navigation are also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application Ser.No. 60/508,723 of Ray Merewether et al. filed Oct. 4, 2003, and entitled“Multi-Sensor Mapping Omnidirectional Sonde and Line Locators andTransmitter Used Therewith,” and U.S. Utility patent application Ser.No. 10/956,328, which was filed Oct. 1, 2004, of Ray Merewether et al.entitled “Multi-Sensor Mapping Omnidirectional Sonde and Line Locators,”now issued as U.S. Pat. No. 7,336,078 granted on Feb. 26, 2008, theentire disclosures of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to electronic systems and methods forlocating buried or otherwise inaccessible pipes and other conduits, aswell as cables, conductors and inserted transmitters, by detecting anelectromagnetic signal emitted by these buried objects.

BACKGROUND OF THE INVENTION

There are many situations where is it desirable to locate buriedutilities such as pipes and cables. For example, prior to starting anynew construction that involves excavation, it is important to locateexisting underground utilities such as underground power lines, gaslines, phone lines, fiber optic cable conduits, CATV cables, sprinklercontrol wiring, water pipes, sewer pipes, etc., collectively andindividually referred to hereinafter as “utilities” or “objects.” Asused herein the term “buried” refers not only to objects below thesurface of the ground, but in addition, to objects located inside walls,between floors in multi-story buildings or cast into concrete slabs,etc. If a back hoe or other excavation equipment hits a high voltageline or a gas line, serious injury and property damage can result.Severing water mains and sewer lines leads to messy cleanups. Thedestruction of power and data cables can seriously disrupt the comfortand convenience of residents and cost businesses huge financial losses.

Buried objects can be located by sensing an electromagnetic signalemitted by the same. Some cables such as power lines are alreadyenergized and emit their own long cylindrical electromagnetic field.Other conductive lines need to be energized with an outside electricalsource having a frequency typically in a range of approximately 50 Hz to500 kHz in order to be located. Location of buried long conductors isoften referred to as “line tracing.”

A sonde (also called a transmitter, beacon or duct probe) typicallyincludes a coil of wire wrapped around a ferromagnetic core. The coil isenergized with a standard electrical source at a desired frequency,typically in a range of approximately 50 Hz to 500 kHz. The sonde can beattached to a push cable or line or it may be self-contained so that itcan be flushed. A sonde generates a more complex electromagnetic fieldthan that produced by an energized line. However, a sonde can belocalized to a single point. A typical low frequency sonde does notstrongly couple to other objects and thereby produce complex interferingfields that can occur during the tracing. The term “buried objects” asused herein also includes sondes and buried locateable markers such asmarker balls.

Besides locating buried objects prior to excavation, it is furtherdesirable to be able to determine their depth. This is generally done bymeasuring the difference in field strength at two locations.

Portable locators that heretofore have been developed lack thefunctionality needed to quickly and accurately locate buried utilities.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a portable locatorincluded at least one articulatable antenna support structure thatpermits a spacing or orientation between at least two antenna elementsto be varied.

According to another aspect of the present invention, an antenna arrayincludes an enclosure with a plurality of antenna elements in the formof coils. The array includes at least three mutually orthogonal coilsmounted adjacent corresponding surfaces of the enclosure. The anglesbetween the axis of each coil and a common antenna support structure aresubstantially equal.

According to another aspect of the present invention, a portable locatorinterprets low frequency emissions for navigation and low or highfrequency emissions for locating buried objects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a tripod locator with its legs extended.

FIG. 2 illustrates the tripod locator with its legs closed.

FIG. 3 is a side elevation view illustrating the tripod locator with itslegs open, sitting on the surface of the ground and illustrating DopplerNAV based in the main case of the unit.

FIG. 4 illustrates an antenna node with six coils.

FIG. 5 illustrates an antenna node with three coils.

FIG. 6 is a perspective view of a coil gradient pair locator.

FIG. 7 illustrates an antenna node gradient pair.

FIG. 8 is a block diagram of a circuit for processing signals from anantenna node.

FIG. 9 is a block diagram of a portable locator.

FIG. 10 is a block diagram of a locator system with enhanced processingcapabilities.

FIG. 11 is a flow diagram illustrating a method of transmitter locationcalibration.

FIG. 12 illustrates an underground utilities map generated with alocator.

FIG. 13 illustrates an alternate tripod locator configuration in anunfolded state.

FIG. 14 is a view of the tripod locator of FIG. 13 in a folded state.

FIG. 15 illustrates a locator with two nodes in its extendedconfiguration.

FIG. 16 illustrates the locator of FIG. 14 in its collapsedconfiguration.

FIG. 17 is a perspective view of an orthogonal sonde array.

FIG. 18 is a sectional view through FIG. 17 showing the construction ofthe array.

FIG. 19 is a block diagram of a navigational sonde beacon.

DETAILED DESCRIPTION

The entire disclosure of co-pending U.S. patent application Ser. No.10/308,752 of Mark S. Olsson, filed Dec. 3, 2002, and entitled “Singleand Multi-Trace Omnidirectional Sonde and Line Locators and TransmitterUsed Therewith,” is hereby incorporated by reference.

The present invention provides an improved cable, pipe and sonde (dipoletransmitter) locator that uses spatial measurement techniques for moreefficient and accurate locating. The locator overcomes two key locationproblems. The first problem relates to locating sondes. Dipoletransmitters (sondes) have a complex shaped “bar magnet” type of field.Measuring the flux vector (field line intensity and direction) does notindicate to the operator where the sonde is located underground. Anadditional complication in one of the properties of this type of fieldsource is that the intensity in the direction of the N-S poles of the“bar magnet” is twice that of the field at the same distance in thedirection of the equator (90° to the axis of the dipole). A furthercharacteristic of a dipole sonde transmitter is that the field intensity(at distances large with respect to the length of the dipole source, farfield) decreases as the inverse of the third power of distance (1/R³).It has been standard locating practice for many years to assume that thesonde lies approximately horizontal to the earth's surface, and to findtwo locations (nulls), above ground where the field lines are verticalthat occur in a plane aligned with the axis of the sonde approximatelyequally spaced on the surface of the ground. Using a single coil,horizontally oriented antenna these locations manifest themselves assignal nulls, often just called nulls. Much later art refers to thesepoles as “locate points.” Using the same antenna, the area directlyabove the sonde is measured as a signal peak so long as the axis of theantenna is approximately aligned with the axis of the sonde. This isoften referred to as locating a sonde using peaks and nulls.

While useful, this is an indirect locating approach. The poles or nullsare really artificial points that are not of direct interest. The goalof locating is to locate the sonde itself. In horizontal drillingapplications as well as conduit locating, it is generally a goodassumption that the axis of the sonde is nominally horizontal withrespect to the earth's surface. The actual surface of the ground, abovethe sonde of course, can often be highly sloped. In locating drainlines, however, the piping often transitions vertically for shortdistances, making sonde location by the method of peaks and nullsimpossible. Vertical sondes as well as highly sloped conduits or steephillsides make sonde locating by the peaks and nulls method difficult,inefficient and often inaccurate.

The problems associated with cable and pipe locating are different. Atfirst blush, the problem appears much simpler since straight linesources exhibit a simple cylindrical field shape where the intensity ofthe field diminishes as a simple inverse relationship to distance (1/R).The shape of the sonde field is far more complex, but it is typicallydiscrete and largely undistorted. Low frequency sondes in particularhave a very small amount of coupling into other adjacent fieldconductive objects. In the cable locating world, however, to somedegree, everything couples to everything, and cables are often notstraight and singular. In many cases, field shape (the direction of theflux vector at a single point in space) correlates rather poorly withthe actual positions in the ground of the objects of interest. Thepresent invention allows the three dimensional properties of the fieldto be measured, offering large improvements in the accuracy andefficiency of locating cables and pipes in the congested environmentstypically encountered.

As an improvement to existing locators, our improved locator measuresthe electromagnetic field emitted by a cable, pipe or sonde throughout aspatial volume, in order to determine not only the direction of thefield lines but also the gradient of field intensity. This improvedlocator collects a much greater amount of measured field data in realtime, this combined with more advanced processing techniques can also beused to directly calculate an estimated object geometry and position.

Existing locators measure field direction and rely on the operator tomove the locator within the work area to search for areas of greaterfield intensity. One embodiment of our improved locator can directlymeasure direction from the operator, in which the field intensity isincreasing. Another embodiment of our improved locator continuouslydetermines its position relative to the search coordinate system (localnavigation), thus allowing field measurements in time to be spatiallyrelated one to another. This allows the locator to also determine thethree dimensional properties of the field using as few as three fieldsensors while tracking the movement of these sensors through thelocating workspace. Another embodiment of our improved locator combinesthese techniques of using a volumetric antenna array, combined withlocal navigation, to allow even greater locating accuracy and efficiencyto be achieved.

Multiple antennas are placed within a three dimensional volume to allowthe volumetric properties of the field to be measured. Measurement ofthe volumetric properties of the field allows the true position of theobject of interest to be more accurately estimated. The antennas can beany device capable of sensing an electromagnetic field, examples includebut are not limited to air coils, coils wound on cores typically iron orferrite, or other magnetic field sensors, GMI, GMR, flux gate, etc.

The antennas can be clustered into mutually orthogonal sets of threesensors. Due to the availability of a large amount of processing power,however, there is no requirement for the sensors to be grouped inorthogonal sets, and these antenna sensors can be alternativelydistributed individually in space and a resultant field solution can becalculated. Various calculation techniques can be used, one example ofwhich is maximum likelihood.

A practical problem to overcome is that locators are portable hand heldinstruments and a large enclosed volume can be expensive to manufacture,and impractical to store and carry. Two preferred embodiments of ourinvention offer solutions to this problem.

One preferred embodiment of our locator uses an improved novel antennasupport geometry based upon a sphere. We will refer to this as agradient antenna node. Volumetrically, in terms of surface area tovolume, a sphere is the most efficient geometry. In the role as amounting structure, surface area relates to the weight of the completestructure. As the size and numbers of antenna support structureincreases, the importance of keeping weight at a minimum becomesparamount in a portable, hand held instrument. In this embodiment weemploy large diameter air coils orthogonally placed on the insidesurfaces of a sphere. These would be typically manufactured as twomating hemispheres. Geometrically, this is equivalent to the sixorthogonal faces of a cube enclosed within a spherical shell. Coilsmounted on the opposite faces of the virtual internal cube are coaxial.The signals from coaxial coil pairs can either be summed to optimize formaximum sensitivity or subtracted (differenced) so that the vectorcomponent of the field intensity gradient in the direction of the coilaxis can be determined. It is not necessary to populate all six internalpositions. To reduce cost and weight, only three orthogonal positionsneed to be populated with coils. An alternate embodiment populates oneinternal orthogonal set of three coils with wire sizes and turn numbersoptimized for higher frequencies (typically fewer turns) and the otherset with coils optimized for a lower frequency range.

The enclosure for the coils need not be exactly spherical. In analternate embodiment, the faces of a sphere corresponding to the planeof each mounted coil are flattened into a more cubic configuration. Anexternally spherical configuration will be generally stronger, but asphere with flattened faces will be more compact. A further advance ofplacing the coils on the inside surfaces of the supporting sphericalshell provides a maximum amount of internal space for additional sensorsand supporting electronics.

Referring to FIGS. 1-3, a tripod locator 500 incorporates three antennasensors or nodes 520, 522 and 524 mounted on the ends of foldable orarticulatable legs or supports 510, 512 and 514 allowing these sensorsto be positioned within a three dimensional volume. A fourth sensor ornode 526 is positioned near the upper support point of the tripod. Theantenna node 526 is carried on a central leg or support 516. In theparticular embodiment illustrated in FIGS. 1-3, the antenna nodes areapproximately positioned at the corners of a tetrahedron. At least twoof the three legs of the tripod are foldable to allow for use or storagein a more compact configuration. The locator 500 is designed to becarried by hand by an operator in the field at a work site. As such, theantenna sensors must not interfere with the operators ability to walksafely, unimpaired. Preferably, one leg opens rearward within the planeof the handle of the instrument, while the other two legs of the tripodopen forward in front of the operator, approximately 60° from thiscentral plane, to the operator's left and the right. This configurationis easy for the operator to carry and operate.

The upper ends of the supports 510, 512 and 514 are pivotally connectedto a common pivot base 530. Snap retainers 528 on support 516 releasablyhold supports 510, 512 and 514. The tripod configuration has theadditional advantage of allowing the operator to place the locator 500in an upright fixed position on the ground. In this mode of operation,the locator can monitor and track the movement of signaling devices, forexample, a dipole transmitter located in a horizontal drilling head.This also allows the locator to remain in a fixed location during aperiod of time during GPS position acquisition.

The tripod locator 500 of FIG. 1 could be used to measure the slope of adrain line by positioning the locator near the line to be measured andpushing a sonde along the pipe through this region. The fixed locatorcan precisely track the sonde's movement in three dimensions and therebydetermine if the line has proper drainage slope. The tripod locator 500(FIGS. 1-3) has a housing 32 with a handle portion 42 that encloses mostof its electronics. The housing 32 has a handle portion 42 that extendsbetween a rear battery enclosure portion of the housing 32 and a forwardmain portion of the housing 32. The supports 510, 512, 514 and 516connect to the main portion of the housing 32. The batteries serve tocounterbalance the supports. A keypad 106 and display 48 are mounted inthe housing 32 as best seen in FIGS. 1 and 2. FIG. 2 shows the locatorof FIG. 1 with the legs shown closed for storage or transport, or foruse in a more compact form. As best seen in FIG. 3 the sensors 520, 522and 524 rest on the surface of the earth 532. Doppler radar emissionsare illustrated diagrammatically in FIGS. 3 and 534. Laser beams areillustrated diagrammatically at 536, ultrasonic waves are illustrateddiagrammatically at 538, and optional imaging is illustrateddiagrammatically at 539.

The performance of our locating system can be further improved andenhanced by adding over the ground navigational capability. Variousnavigation techniques can be employed. Examples include, GPS, DGPS,inertial navigation, optical flow (using a camera linear array, opticalmotion processor or other optical technology), Doppler techniques, andthe use of navigational reference beacons. With sufficiently accuratenavigation, a local map of the detected objects can be stored in thelocator, either for use by the operator in the field, or for downloadinto a data system such as a GIS (Geographic Information System). Kalmanfiltering or similar techniques can be used to integrate variousnavigational sensors for improved accuracy. For accurate utilitymapping, a spatial resolution of substantially less than one meter isneeded and will often not be available from GPS (shadowed by or insidebuildings, under tree cover, etc.). A more accurate and robust localnavigation scheme is needed to complement GPS techniques.

With a sufficient number of non-coplanar, spaced apart antennas, andsufficient signal strength, the position and orientation of a sonderelative to the locator 500 can be accurately determined. The reversecan also be true. If one or more sondes are fixed and not moving, thesesondes can be used as reference beacons to locally navigate the locator500 with respect to the position of one or more fixed beacons. Whenusing multiple sondes for locator navigation, each sonde can operate ata unique frequency or use another coding method, e.g., TDM, to allow themapping locator to uniquely distinguish the signal from each sonde.Lower frequencies are desirable to avoid signal coupling into nearbyobjects and resultant navigational errors. An over-sampled system isdesirable. Examples include: a single vertical sonde plus a compass; twovertical sondes, working on one side of a base line running from onesonde to the other; three vertical sondes in an approximate triangularconfiguration; one or more pairs or triads of collocated horizontallyoriented orthogonal (crossed) sondes.

A GPS can be integrated to be part of the navigational sonde beacon. Iftwo beacons are used these can be used in “leapfrog mode” to keep theposition of one tied to the next. The GPS positional information can betransmitted directly via radio or other means either to the locators orthe drilling controller at the drill rig control station. Alternately,the GPS or other positional information can be encoded or modulated ontothe transmitted sonde Beacon signal. The relative positional informationchanges very slowly (drift), or intermittently when the beacon orlocator is moved to a position, so the needed data rates for updatingchanges are low. Beacons that transmit intermittently conserve batterypower.

Similar to how an optical mouse operates, an image can be used to trackthe movement of the locator over the ground. Additionally, images of theground can be stored for later retrieval. Additionally, a largephotomosaic of the work area can be assembled from these images, eitherin real time or post-processed. Image collection can be employed withnon-optical flow navigation schemes to provide information forassembling individual images into a large mosaic of all areas traversedby the operator.

Capabilities that can be added or associated with navigation includes:

-   1) Ground Penetrating Radar (GPR);-   2) Acoustic tomography (geophones in leg tips, using drilling noise    to build obstruction images); and-   3) Conductance tomography (electrical conductance probes in leg    tips).

A two axis tilt sensor is preferably included in the locator 500. Athree axis tilt sensor could be advantageous in some applications. Anearth referenced magnetic compass may also be desirable. The user placesthe tripod locator 500 above the target drilling location and simplysteers the drilling system towards it. Multiple receivers can be used tolimit the amount of operator repositioning activity. The transmitter inthe drill head can be used as a continuous position reference whenmoving a receiver. Importantly, this system does not require pitch oryaw information to be transmitted from the drill head. Nor does itrequire that the drill string move forward to allow multiplemeasurements to be made with the drill head transmitter at variouspositions in space to reduce the number of unknowns to allow thetransmitter position to be determined. Nor does it require that thesignal strength of the transmitter be known. Each receiver can have anIP address, and utilize a longer range TCIP type of communicationprotocol. This facilitates the use of a conventional portable computeras a drilling control console.

Our multi-sensor locator 500 allows the drilling control system tocalculate and steer to any target point in space within the detectionrange of the unit compared to the “flux pathline steering procedure.”Longer legs and larger opening angle can be used for a fixed mount typeof receiver. Greater separation can yield greater accuracy. Ourmulti-sensor locator 500 can easily establish an accurate initialmagnetic bearing of the drill at the beginning of the drilling process.Crossing cables can be located and displayed similarly by conventionaland direct measurement techniques. Our invention allows use of lowercost transmitters that do not require pitch and roll information.

Separate frequency transmitters can allow greater pitch measurementresolution. Using the known distance, e.g., ten feet, between thetransmitters emitting at slightly different frequencies, the locator 500can compensate for skin effect errors. The two frequencies should besimilar so that propagation in the earth is effectively identical. Alsoif pitch and roll data do not need to be transmitted, then very lowfrequency sondes, with essentially no skin effect error can be utilized.The spaced apart sensor array of FIG. 1 also allows skin depthcorrection. Separated sondes also have the effect of increasing thelocate range of the drill string. Great pitch precision can be obtainedwith a good separation. It is possible to duty cycle the trailing sondeto extend battery life and make it easier to justify having a secondlocation for a sonde in the drill string. Our tripod locator 500 withtilt sensors, maps the local slope of the surface of the ground when itis placed upon the ground to make measurements.

GPS accuracy and coverage in urban settings may be substantiallyimproved by providing artificial GPS satellites (pseudolites) that canbe placed at corner markers, permanent natural features, buildingentrances, or other semipermanent features. Pseudolites may also beplaced in relatively open areas at a location surveyed by DGPS/WAAS ortraditional means to tie a mapping locator survey into more globalcoordinates. A pseudolite or theodolite total station may include imageacquisition and processing means so that a similar pseudolite ortheodolite with image acquisition ability may be accurately positionedin the same place in a separate survey.

In a region of space, free of electrical current and approximately freeof displacement current, the curl of the magnetic field is alsoapproximately zero. In that case, only five components of the magneticgradient and three magnetic magnitudes at each frequency are needed toclosely approximate the actual field. In a region of space near manyconductors carrying synchronous currents, more components are needed tocharacterize the field. In particular, it is reasonable to map howquickly the first order gradients are changing in three dimensions. Suchmeasurements may be made by providing first order gradiometers inantenna nodes as opposed coil pairs, providing coil pairs withindependent channels and performing addition and subtraction inhardware, firmware, and/or software, and/or providing precise threedimension navigation of simple multiple coil antennas. If each node inthe tetrahedral tripod locator contains coil pairs capable of beingprocessed as first order gradiometers, the differences in the outputs ofsaid gradiometers and the distances between then may be used to estimatesecond order gradients. Such gradient information is useful forcharacterizing horizontal, tilting, and even vertical pipes andutilities. Maps of magnetic intensity, first order gradients, and higherorder gradients to measure field curvature may advantageously be used tocharacterize regions with multiple electromagnetically interactingutilities and pipes. It is not necessary to resolve the outputs of theplurality of magnetic sensors into gradients and orthogonal magnitudecomponents to effect an inversion solution of the location of the pipesand utilities. Methods such as the Nelder Mead simplex algorithm canwork directly in the vector space of coil output voltages and phases.Displaying maps of higher order gradients will help an operator of themapping locator interact with the device to resolve particularlycomplicated situations.

Image based navigation and path documentation may be accomplished at arelatively low frame rate by cross correlating successive images. If thecross correlation peak is near the origin, one of the frames may bediscarded to save memory. Alternatively, the two frames may beregistered and then merged to enhance resolution. Such image processingmay be accomplished in a combined image acquisition and image processingdevice or by a separate processor operating on the output of an imagingdevice. Methods of image enhancement may include but are not limited tobicubic interpolation, directional edge interpolation, or interpolationby anisotropic diffusion. Image frame sequences may be used to tiesurvey areas in navigation satellite shadows and multipath regions totraditionally surveyed corner marker grids and/or multipath free GPSlocations.

The amount of time and memory required to cross correlate images fornavigation purposes may be substantially reduced by cross correlatingsub-areas, such as the corners, of one image with the other image.Selection of sub-areas in both images for cross correlation may beguided by raw information from navigation sensors or from the output ofa Kalman navigation system. The location of the peaks in the crosscorrelations of successive images may in turn be fed into a Kalmanfilter as estimators of the mapping locators trajectory and orientationchanges in three dimensional space.

Cross correlation may be performed on pixels. At high frame rates,sub-pixel resolution may be obtained by the methods of optical flowanalysis. Alternatively cross correlation may be performed on objects.Objects may be identified by repeatedly applying erosion and dilationoperators to image data, color matching, or by texture analysis usingmethods such as windowed Fourier transforms, Short time FourierTransform (STFT), or Gabor transform analysis.

Stored image data may be transferred to another processor on anexcavator or other earth working machinery for direct viewing by theoperator as verification of correct location and orientation fordigging, drilling, and/or grading. Overlaying one or a plurality ofimages as partially transparent layers over an opaque comparison imageis a display mode usable by machine operators for position andorientation registration. Cross correlation of transferred images withstored and current image data allows registration of apparentlyfeatureless surfaces such as asphalt or concrete and apparently randomsurfaces such as sandstone or fine grain rocks.

Other instruments such as Ground Penetrating Radars (GPR), gravimeters,seismometers, geophones, magnetometers, gradient magnetometers, gaschromatography mass spectrometers, gradient magnetometers, conepenetrometers, and electromagnetic induction detectors may also beequipped with Kalman navigation systems and optical imaging devices.Stored image data may be transferred between such devices to coordinatemapping of subsurface features to hundreds of meters in depth.Alternatively, images, navigation data, and measurement data may betransferred to an independent processing means for data fusionoperations. Such fused data is useful for environmental site remediationplanning and assessment, construction planning, and utility planning.

A preferred embodiment of our design aligns the long axis of theexternal mounting support with two of the corners of an internal“virtual cube.” This provides space to allow structure to be added tothe spherical shells for mechanical support as well as allow wires orother data transmission means to enter the interior space of theaspherical shell without interfering with the coils. If the coils areslightly reduced in size, this geometry also allows a supporting memberto pass through the shell for greater mechanical strength, or to allowan additional antenna set (node) to be mounted along another section ofthe same support member. This further has the important property thatall of the coils within the spherical shell are exactly at the sameangle with respect to the long axis of the supporting member. This canbe extremely important in terms of facilitating the calibration of thesensitivity of each coil relative to the others in the array. The matingplane of the two hemispherical shells is preferable normal to the axisof the antenna node support member, but many other orientations can alsobe used.

As illustrated in FIGS. 4 and 5, the spherical enclosure can be designedas two mating halves with a planar interconnecting element placedapproximately at the mating plane of the two halves. The interconnectingelement is preferably a printed circuit board 544. This allows the useof a socket and pin interconnect means to each hemisphere if the coilsare mounted directly to the insides of the hemispheres. The coils can bemounted to a separate internal structure enclosed by approximatelyhemispherical shells.

In the node 520 illustrated in FIG. 4, six coils are arrangedorthogonally in alignment with the sides of a cube so that they can fitwithin a spherical shell of halves 540 and 542 connected to a tubularnode support 510. The coils include a first opposing pair of coils 546 aand 546 b, a second opposing pair of coils 548 a and 548 b, and a thirdopposing pair of coils 550 a and 550 b. In the node 552 illustrated inFIG. 5, three coils 554, 556 and 558 are orthogonally arranged withinthe spherical shell made of halves 540 and 542. In both the nodes 520and 552, the angle between the axis of each coil and the axis of thesupport 510 is substantially the same. The coils illustrated are allidentical and deformable to nest inside each other. A first coil isinstalled in the desired location, and a second, deformable coil is theninstalled orthogonal to the first so that it crosses over (or inside) ofthe first coil. A third, identical coil is installed over the first twoso that it is orthogonal to both of the first two. There are two keyadvantages to this approach; one is lower costs due to using a single,unique part; and two, and even more importantly, all of the coils areelectrically identical and have essentially the same frequency response.The deformation introduces only a very small difference in thesensitivity of each coil.

FIGS. 6 and 7 show a version of the antenna node 620 in which the facesof the sphere corresponding to the plane of each mounted coil have beenflattened into a more cubic configuration. An externally sphericalconfiguration will be general stronger, but a sphere with flattenedfaces, with be more compact. In FIG. 6, a coil gradient pair locator 600has the node 620 attached to the lower end of an elongate support whoseupper end is attached to the housing 32.

In FIG. 7, the node 620 that is connected to support 610 includes shells622 and 624, secured via top retainer 630 and screws 632 and 634. A PCB636 supports circuitry. Inner supports 626 hold coils of pairs 640 a,640 b, 642 a, 642 b and 644 c, 644 b. A large transmitter/receiver coil628 is also included.

FIG. 8 is a block diagram of a circuit 800 for processing signals froman antenna node. Signals from an antenna array 802 are fed through aninput protection circuit 804 and to mixers 806 and to a high resolutionmulti-channel analog-to-digital (A/D) converter 808. The mixers 806receive input from a numerically controllable oscillator (NCO) 810controlled by a signal routing, or communication and address packingcircuit 812 connected to a processing system 814 inside the housing 32.The NCO 810 receives timing signals from a clock source 815. The NCO canbe a traditional oscillator, or a temperature compensated oscillator oran oven controlled oscillator, or could be referenced from a portableatomic clock module, or a clock derived from a GPS. This would allow fornarrower filters, better phase comparison, lower drift, etc. Local poweris provided through a circuit 816 and additional sensors 818 provideinput into the A/D converter 808.

FIG. 9 is a block diagram of a portable locator in accordance with thepresent invention. An antenna assembly 902 includes a left front antennanode 904, a right front antenna node 906, a central antenna 908 and arear antenna node 910. The antenna assembly can include an optional topcentral antenna 911. Signals from the antenna assembly 902 are fed to anode communications processing circuit 912 which in turn feeds a sensorprocessing circuit 914. The sensor processing circuit 914 also receivesinputs from an auxiliary sensor port 915. Both the node communicationsprocessing circuit 912 and the sensor processing circuit 914 are locatedon a main printed circuit board 916 located within the housing 32.Batteries 918 mounted within the housing 32 are connected to a systempower supply 920 on the main circuit board 916. The main circuit board916 also supports general system sensors such as temperature, batterylevel, light level, etc., which are indicated diagrammatically at 922.The main circuit board 916 also includes system RAM 924, user flash ROM926, and system flash memory 928. Indirect navigation modules 930 suchas GPS, optical, radio triangulation, etc., are also supported on themain circuit board 916. Physical navigation modules 932, such as acompass, inclination/tilt sensor, inertial/gyro Doppler radar,altimeter, etc., are also supported on the main circuit board 916. Akeypad and light sensor circuit 934 are mounted in housing 32. Triggerand reserve push buttons 936 are also mounted in the housing 32. Thecircuit 934 and the buttons 936 are connected to an input/output controlcircuit 938 mounted on the main circuit board 916. A communications port940 is also connected to the input/output control circuit 938. Agraphical liquid crystal display 942 mounted in the housing 32 is drivenby a user interface and control processing circuit 944. A radio link 946is also connected to the main circuit board 916. An active marker drivecircuit 948 is connected to the main circuit board 916 and is used toexcite buried markers. An audio amplifier 949 on the main circuit board916 can drive either headphones (not illustrated) through a headphonejack 950 or a speaker 952.

FIG. 10 is a block diagram of the locator system with navigationalcapabilities. An indirect navigation module 1000, an environmentalsensor 1002 and a terrain sensing module 1004 provide input to a vectornormalization module 1006. Vector normalization module 1006 receivesinput from an antenna assembly 1008. The locations of undergroundutilities are indicated on a mapping display 1010.

A fundamental problem in portable locators is that the user's motion isconvolved with the received signal. To perform near optimal filtering,the effects of motion need to be deconvolved before applying the nominalmatched filter or, alternatively, the structure of the filter needs tobe substantially modified to incorporate the motion inputs.

In a typical usage, a portable locator is swung side to side or aroundin a large arc to find an initial direction to a buried utility. Even inthe case of following a buried utility, small deviations in path mayresult in phase reversals of the signals in those antennas in antennaassembly 1008 which are aligned so that their sensitive axis isapproximately perpendicular to the local magnetic flux lines. Ingeneral, the sensitivity of any antenna is proportional to sine of theangle between its sensing axis and the local flux lines. If the sensingaxis is aligned with local flux lines, the sine of the angle is zero andthat particular antenna is insensitive. Prior art portable locators havenot attempted to correct for this effect.

The first order correction is to form a times series of vectors with theoutputs available from an single node in the antenna array 1008 and touse angle change information from the physical navigation sensor suites1002 and 1004 to rotate the time series of vectors into a rotation freeframe of reference before those signals are applied to filters. Thefilters used depend on the signal being traced. Traditional portablelocators have used very narrow band filters. User motion has placed alimit on the minimum bandwidth in the filters. Rotating the antennaoutput vector time series allows the use of narrower filters forimproved detection range. At large ranges, the signal varies very slowlywith changes in range and a narrower filter is more desirable. In abroadband locator, very similar issues are relevant. If the user'srotational motion is significant on the time scale of the code lengthused, the matched filter will see significant de-correlation and a lossof output amplitude will occur. Again, if the signals are rotated to astationary frame before application to the matched filters, significantimprovement in detected signal strength is seen. In a preferredembodiment, each node in antenna array 1008 produces three orthogonalsignal components. For each of these nodes the first step is to rotatethe three signal components into components aligned with the system axisused by the navigation sensors 1002 and 1004. This is an ordinary matrixmultiplication. The second step in the preferred embodiment is to useroll, pitch and yaw signals for the navigation suites 1002 and 1004 toform a matrix operator that rotates signals in the portable locatorframe of reference into an earth referenced frame. In most applications,the utilities, navigation beacons, and jammers are stationary in theearth-referenced frame. Signals referenced to that frame may beadvantageously processed by considerably narrower filters. These matrixmultiplications may be performed in block fixed point or floating pointin a general purpose processor, digital signal processor, or customhardware accelerator. Subsequent filtering and processing in block 1016may be in the same physical processor in a second DSP.

A second level of correction is possible. In any particular orientationof the antenna assembly 1008, some of the antennas will be closer to theburied utilities, some will be closer to navigation beacons, and somewill be closer to inferring sources or “jammers” than other antennas.Again, user motion will modulate, or be convolved with, the desiredsignals from the utilities and navigation beacons and will also modulatethe interfering sources. This modulation spreads the spectrum of boththe desired and undesirable signals and makes their separation in thefollowing filters more difficult. The translational and angularinformation from physical navigation sensor suites 1002 and 1004 may beused with the known physical configuration of the assembly 1008 totransform the signal vectors into a quasi-stationary frame before beingprocessed by the matched filters. This procedure minimizes the amount ofbandwidth spreading and signal distortion caused by user motion which inturn allows better resolution of the utility and beacon signals andrejection of the interfering signals.

FIG. 11 is a flow diagram illustrating a process of transmitter locationcalibration. In the case that more than one sonde beacon is used in alocal survey, the beacons can be provided with a mechanism to selfcalibrate their relative positions. Each beacon needs a receivingantenna array much like that used in the portable locator antennaassembly 1008. Each beacon can also contain a GPS receiver. As eachbeacon is placed in the field and turned on it begins to integrate thetime series of GPS fixes it receives. In addition, the beacons will turnon their internal receiving antenna nodes to calculate the relativepositions of the other beacons. The integrated GPS and relative positioninformation can be modulated onto the beacon signals, modulated onto alocal infrared or radio net, or incorporated into pseudolitetransmissions for use by other beacons and the portable locator. Thebeacon network self calibration can proceed in the absence of a portablelocator. A least squares fit of all of the integrated GPS locations andrelative position information from the beacons is saved with time stampsfor subsequent use by a portable locator. Once mapping location isbegun, further refinement of the relative position least squares fit isperformed using data on the position of each of navigation beaconsrelative to the portable mapping locator.

One important problem our mapping locator solves is the ability torecord or transmit utility positional data collected in the field sothat this information can be used to create, update or improveGeographical Information System (GIS) maps. FIG. 12 illustrates anexample of such a map 1200 that can be generated directly in the mappinglocator or built from recorded data during incorporation into a mappingsoftware utility.

In FIG. 12, curb edges 1202 of an intersection of roads are representedgraphically. The same is true of a fire hydrant 1204, stop sign 1206,North direction as established by a sonde placement 1208, a survey mark1210, another sonde placement 1212, a utility pole 1214, an electricalbox 1216, a phone box 1218, a fiber optic terminal box 1220 and a gasmeter 1222. Gas, electric phone and fiber optic lines running parallelwith the streets are also represented.

Utility owners have maps indicating the locations of their utilities,both buried and overhead. These maps are often incomplete or inaccurate,at least to the level needed to support construction activities.Existing requirements for locating utilities prior to digging andconstruction are limited by the capabilities of the instrumentsavailable to perform these locates. For example, one existingrequirement is to place marks within two feet of the true position ofthe buried utilities, with no requirement at all to note depth. While onsite, the personnel performing the locate typically use colored paint tomark hard surfaces or stakes with colored flags to mark soil. This istypically the sole method by which the results of the utility locate arecommunicated to the construction personnel. However, stakes are easilymoved, and paint is considered unsightly and may last a short time.Currently, there is no easy way to either audit the locating process orprovide a permanent electronic data record of each locate, and repeatconstruction typically requires repeat locates. Because millions oflocates are performed in the United States each year, it would be ofgreat value if this information was available to continually improve GISmaps. Providing construction personnel with a map of the site, eitherprinted or even downloaded into a mapping locator onsite for use duringthe construction process, would reduce errors and subsequent damage andinjury.

Typically, conventional GPS is only accurate to 3-10 meters. This iscompletely inadequate for buried utility construction activities whererelative positions between buried utilities are often less than onemeter, and utility crossing separations may be on the order of tens ofcentimeters vertically. Greater accuracy is needed. In urban settingsaround large structures, GPS may not be available, and accuracy may befurther distorted by multi-path effects. DGPS techniques using postprocessing or more sophisticated field equipment can, in some cases,increase precision to decimeter levels, but it appears unlikely thatsuch techniques can be readily employed in many typical urban settings.GPS techniques will play an important role in the use of our mappinglocator to tie the locate into a global coordinate system to varyingdegrees of precision.

A related problem is that locating errors are common, and a great dealof property damage and injuries related to utility strikes duringconstruction occur each year. The various configurations of our mappinglocator offer many improvements and will greatly improve locatingaccuracy. The old saying “garbage in, garbage out” applies with respectto putting information into databases. It is important that accurateinformation is placed into GIS systems, or these errors themselves canlater result in damage during construction, if relied upon. Perhaps ofequal importance is being able to estimate the quality of data recorded.Since it is collecting data over an area in a known spatial relation,our mapping locator allows the accuracy of the data to be estimated, andpotential interferences and distortions to be readily seen of thepatterns in the data recorded. Undistorted data will match a cylindricalfield model for a straight conductor, but distorted data will not. Dataqualification can be done in real time in the field, post processed inthe field at the end of the locate when a complete data set isavailable, or post processed at a later time, such as during the processof GIS incorporation or merge. Knowing when the collected data isinvalid or likely to be in error due to non-simple multiple fieldsinteracting is critically important when determining whether othertechniques such as alternate (typically lower) frequencies, commonground isolation or even potholing, may be required to establishaccurate utility location. Perhaps the greatest pitfall of non-mapping,non-omni-directional locating techniques is that the operator may nothave any indication that the indicated locate is bad due to fielddistortion or interference. Mapping cannot completely eliminate thisconcern, but it can dramatically reduce this uncertainty.

Locating utilities with respect to other permanent, identifiable objectsin the area of concern is extremely useful. It is important tounderstand that our mapping locator can also map objects that do notemit any signals, which we is referred to as “tracing mode.” Verysimply, this is a mode where the user requests the mapping locatorrecord a point or track and then optionally tags that point or trackwith some identification. Reference objects in the field might includecurb edges, sidewalk or pavement boundaries, signs, poles, utilitycabinets, building corners, fire hydrants, survey markers, etc. Theability to incorporate into the locating map such other featuresrecorded during the locating process allows the position of the mappedutilities to be referenced at a fine scale to important way points inthe local coordinate system. The trace mapping point can be defined withrespect to the mapping locator in any convenient location. Typically,the trace mapping point would be the center of the mapping locator'slocal coordinate system, and would, therefore, be placed at the centerof the lower antenna node or at the midpoint of a multiple node lowerantenna array.

Some cities have a need to map all of their assets into a GIS database.A mapping locator can collect some of this additional data to arelatively high degree of precision.

Trace mode could be initiated with a trigger pull, a key press, or avoice command or any other input means. Trace object identificationcould be selected from a preset list stored in the instrument. Audiorecorded with a microphone during tracing can assist with taggingobjects later on while incorporating the locate data into a GIS map, andsimply requires that the operator state what was being traced and otherpertinent information, such as pole number or survey marker number.Audio compression techniques or voice recognition techniques can improvethe efficiency of this process. Audio allows more complex information tobe recorded in the field for later transcription, whereas the use of akeyboard can be more efficient during post processing of the locate datain preparation for database merge. Our mapping locator can be designedto accept a keyboard, touchpad, mouse or other device, such as a USBtrackball, for field data entry. Our mapping locator can also bedesigned with an integrated joystick, micro-track ball or personaldigital assistant (PDA) touchpad (Graffiti, hand writing recognition)input to facilitate adding additional information such as street names,job site, employee ID, etc.

Positional data can be recorded continuously throughout the locate. Thestart and endpoints of traced reference objects can simply be tagged insequence as part of this stream of position data. Alternatively,separate data files for each traced object might be created withassociated compressed audio files for later specific identification.Objects are traced in three-dimensions (3D) so vertical relief items,such as poles or vertical building edges, will stand out in the mappingdata set, even without any associated annotation or tag. To allow acomplete audit of the locate, the position of the line transmitterutility connection and grounding points can be measured. The tracingfrequency in use on each utility can, of course, be recorded as a datacomponent of the mapping information. The simultaneous use of multipletransmitters on various utilities as described in our co-pendingapplication (U.S. Ser. No. 10/308,752) can be used to improve theefficiency of the locating process and allow simultaneous locating ofmultiple utilities.

Our mapping locator can also be used as a surveying tool. Depending onthe types of navigational sensors employed, sub-centimeter resolution isreadily achieved. For example, within a range of several meters of athree-axis navigational sonde beacon, a relative resolution on the orderof 1 mm can be achieved. This capability allows 3D points to be quicklyand efficiently mapped. Our mapping locator can operate as a 3Ddigitizer for use in the field. An example usage would be collectingdata on an existing structure for remodel construction where originaldrawings, especially 3D, do not exist.

A navigational sonde beacon can be affixed to a service vehicle, whichis advantageous if greater beacon power is needed to allow larger areasto be mapped. This configuration can also save operator time as nospecific deployment of a beacon is needed if the service vehicle can beparked adjacent to the work area. A roof or bed mount configuration, incombination with a low frequency navigation signal, minimizes fielddistortions due to the ferromagnetic properties of the vehicle. In thismapping configuration, it would be advantageous to incorporate a GPS, acompass and a tilt sensor into the beacon, or adjacently on the vehicle,in a fixed orientation to the beacon. A further enhancement to thisvehicle mounted beacon configuration is to have the mapping locatorsimply stream positional, field vector and target annotation datacontinuously to a data logging system in the vehicle, such as a laptopcomputer.

Similar to tracing mode, our mapping locator can readily incorporatesonde position data into the recorded map, and thereby, map the positionof non-conductive tubes. If a four node mapping locator is used, it canbe set in place and the sonde inserted, moved and mapped through ahollow conduit, such as a fiber optic conduit or a sewer pipe. Usingeither a single or dual antenna node mapping locator, for example, incombination with the three-axis navigation sonde beacon hereindisclosed, the sonde position could be mapped incrementally whilestopped in increments along the conduit. In this instance, the operatorcan be required to move the mapping locator during the process oflocating each increment along the pipe or conduit. If the orientation ofthe sonde is in fixed alignment and the motion of the sonde is purelylinear, then this requirement may be relaxed.

Versions of our mapping locator using four or more antenna nodes cantrack a moving sonde, while said locator is moving relative to a fixednavigation beacon. Tracking a floating sonde would allow the path of aflow line to be mapped. In such a manner the piping from a house to aseptic tank, for example, might be mapped. Municipal sewer systems couldbe similarly mapped, either with a hand carried or vehicle mountedmapping locator tracking a floating or crawler powered transportedsonde. With synchronized clocks, the position of a data collectingautonomous sewer vehicle (ASV) could be tracked allowing untetheredoperation. Simple, low bandwidth data, or messages such as “I'm Stuck,”might be sent with the sonde signal. Even with traditional, tetheredrobotic crawler cameras, more accurate positional and depth informationwould be available if the camera vehicle was tracked with one of ourmapping locators. This is similar to the horizontal drilling problem,but is unique and different in that the position of the moving locatoritself is being mapped while simultaneously mapping the position of afixed or moving sonde.

There are some applications where it is desirable for a vehicle to tracka buried wire. Our mapping locator provides this capability and canprecisely show the offset position of the vehicle relative to the trackwire.

FIGS. 13 and 14 illustrate an alternate configuration of the tripodlocator 700 in which the nodes 720, 722 and 724 are pivotally connectedvia shorter legs 730, 732 and 734 to a pivot assembly 740. The pivotassembly 740 is connected to the lower end of a central leg or support742 that carries two additional vertically spaced nodes 750 and 752. Theupper end of the central support 742 is connected to the housing 32. Theunfolded and folded states of the tripod locator 700 are illustrated inFIGS. 13 and 14, respectively.

FIG. 15 illustrates a locator 100 with a housing 102 that contains mostof the electronics and the upper end of elongate support 104 isconnected to the housing 102. The elongate support 104 has a lowersegment 104 a and an upper segment 104 b which are pivotably connectedby a hinge assembly 106. A first sensor node 108 is connected to thelower end of the segment 104 a. A second sensor node 110 is mountedintermediate the length of the upper segment 104 b. FIG. 15 illustratesthe extended configuration of the locator 100. FIG. 16 illustrates thecollapsed configuration of the locator 100 in which the lower segment104 a and the lower sensor node 108 have been swung upwardly. A clasp112 on the hinge assembly 106 is used to lock and unlock the same.

FIG. 17 illustrates an array 200 of three orthogonal sonde pairs, eachhaving coil wound around ferrite cores. Six molded core forms 210 a, 210b, 220 a, 220 b, 230 a and 230 b comprising the sondes are visible inFIG. 18. Each of the six sondes has an identical configuration. By wayof example, a sonde 250 (FIG. 18) comprises a central ferrite core 212 athat extends within a surrounding cylindrical plastic bobbin 260. Threeother ferrite cores 212 b, 232 a and 232 b are also visible in FIG. 18.A copper wire coil 270 is would around the bobbin 260. The ferrite core212 a has one end pushed against a rubber O-ring spacer 280 within thebobbin 260. The sondes of the array 200 are arranged and connected inthree separate sets, each set being wired in-phase, in serieselectrically they appear as one coil. A center block 291 of magneticmaterial 231 couples each pair of cores together to make themmagnetically longer. A central hub 292 encloses the block 291 and eachof the bobbins such as 260 screws into the same. A portable hand-heldlocator can be constructed that utilizes the sonde array 200. A squarewave is used to drive a tank circuit. A single power amplifier runningat a single frequency switches from coil-to-coil at substantially thezero current crossing points. All of the coils are preferably identical.All the sonde pairs are orthogonal to each other at an equal angle to avertical support (not illustrated) to reduce receiver coil nulling. Thedrive frequency of all the coils is preferably identical. Comparatively,the drive frequency can vary from coil-to-coil by an integer number ofcycles. The drive period may vary from coil-to-coil by an integer numberof cycles, such as 23, 24 and 25 cycles for the three orthogonal coils.The locator can have other sensors, including a tilt sensor, a compass,GPS, etc., to reduce degrees of freedom to enable the requirement thatthe absolute signal strength of the navigation sondes be known, but notrequired. It is possible to suspend the processing of each channel whena signal from that coil is not broadcast.

One aspect of our mapping locator includes an improved navigationalsonde beacon. This beacon operates off a single constant frequencysignal source in an open loop fashion that does not requirecommunication between the signal source and the transmit coil array. Ourimproved beacon uses a single tanking capacitor bank to increasetransmitted magnetic moment. After some predetermined number of signalcycles, this single tanking capacitor bank is switched at or near thezero crossing of current to the next transmit coil in a predeterminedswitching frequency.

A highly advantageous aspect of this invention allows a standard linelocating transmitter to act as the signal source for our improvednavigational beacon. This is possible since this beacon operates at acontinuous output, single fixed frequency, and hence, does not requireany communication between the signal power source and the transmit coilarray.

Greater output power and range at constant input power can be achievedby using larger coils with larger, lower resistance wire. This allowslarger beacons to be used with the same signal power source if greateroperating ranges are needed. An example of one embodiment of ournavigational beacon is shown in FIG. 17. This embodiment uses ferritecores to achieve a compact portable structure, but air coils could alsobe used.

Our mapping locating receiver uses the signal from the navigationalbeacon and needs to be able to determine which interval in timecorresponds to the transmission of signal from a particular coil in thetransmit coil array. One advantageous method of providing a timing indexis to simply turn off the transmission of all of the coils in the arrayfor some brief period of time. While the tanking capacitor can store theenergy in the tank during an off period, this has the undesirable effectof unloading the signal source, thus making it more difficult tomaintain precise output regulation when the load is switched back on. Itis important to the effective operation of the navigation system thatthe output of the X, Y and Z coils are as equal and uniform in time asis reasonably possible, thereby improving the accuracy of navigation.One embodiment of our invention solves this problem by adding a fourthshielded inductor having approximately the same electricalcharacteristics as the three orthogonal transmit coils. During the offperiod, the magnetic energy in the oscillating tank is stored in anon-transmitting inductor. A toroidal inductor would be one suitablechoice as the majority of the field will remain trapped in the core andnot externally radiated. It is only important that enough of the fieldnot be radiated so that the timing signal is clearly discernable by thereceiver.

A mechanical or electrical switch allows adjustment of the totalcapacitance in the tanking capacitor bank. Such an adjustment serves totune the tank to a different navigational beacon channel, allowing twoor more beacons to be used at the same time in the same area. Switchingthe tank to a significantly higher frequency allows this transmit arrayto also be used as an omnidirectional inducing array when searching forunknown buried utilities.

In another embodiment of our navigational beacon, the tanking capacitorcan be switched out of circuit (bypassed) allowing a multi-frequency orrepeating composite waveform to be transmitted. This would typically beemployed in combination with air coils

Referring to FIG. 19, one embodiment of our navigation sonde beacon 300is illustrated and is connected to a line transmitter signal source ortransmitter 302. The line transmitter 300 outputs an amplified squarewave power signal in the approximate range of 10 Hz to 10 kHz fornavigation purposes. Higher frequencies can be used if induction isdesired or not of any concern. An optional edge detector 304 senses theexact timing of the wave form transitions. A cycle counter 306 is usedto determine coil-to-coil switch timing and can be placed at otherlocations within the circuit. A small transformer 308 can be placed inthe circuit to extract power from the signal to power the beacon, oralternatively, a battery 309 can be used. A tanking capacitor bank 310is used in the circuit. The capacitor bank 310 can be bypassed with aswitch 311 or removed, if untanked operation is desirable. The capacitorbank 310 can include switchable capacitive elements to permit the tankcircuit natural frequencies to be tuned to alternate frequencies eitherby the operator via channel selector 312 or under the control of acontrol block (C) 314. For the purposes of sensing and control, thevoltage across the tanking capacitor bank can be measured by a voltagesensing component 316.

In the example illustrated in FIG. 19, one end of each of the X, Y, Ztransmit coils, 318, 320, 322 as well as an optional non-transmitting,null coil 322, are tied in common to one end of the capacitor bank 310.The voltage at this point can swing to high levels, and it is desirableto place switching elements 326, 328, 330 and 332 at the other ends ofthe coils. Current can optionally be sensed between the capacitor bank310 and the coils. The null coil 324 is preferably a toroid. Since it isdesired to switch at the point of zero current where all of the tankenergy is stored in the capacitor and none in the active inductor(coil), a triac is one suitable switching element. If all of the coilsare electrically matched, current sensing and current control is notrequired. However, greater control of the output can be achieved if acurrent sensing element 334 is used. A Hall effect current sensor can beemployed to measure current. A control circuit 336 can optionally beemployed to improve the coil-to-coil transmitted power matching basedupon feedback from current and voltage sensing elements, or optionally,from a B field sensing element 338 (coil, GMR or similar), or both. Asensing element can be placed on each inductor/coil, or a single sensorcan be employed if its response to each coil can be characterized andknown.

The control block 314 is used to control the switch timing. The controlblock 314 is designed to switch one coil out of and the next coil intothe tank circuit 310 when the current flowing through the activeinductor is near or at zero. Where the control circuit 336 is employed,the control block 314 substantially equalizes the current or theradiated measured B field from each coil.

The internal timing between coil-to-coil switching need not be uniform,but must be equal to an integral number of input drive signal halfcycles. A preferred embodiment would have the transmit time of each ofthe X, Y, and Z coils be equal, but utilize a shorter period of off,non-transmitting time during which the null coil was switched in. Thecontrol system works to keep the maximum stored energy in the tankingcapacitor bank constant from cycle to cycle.

The navigation sonde beacon housing would optionally include a levelindicating device such as a bubble level 340 and an optional means forthe user to level the case to true earth horizontal. Two fixedsupporting points and one adjustable supporting point provide a simplemeans to allow the user to level the transmitting array to a knownposition relative to the earth's surface. Additionally, a magneticcompass 342 (electronic or mechanical) can be used to aid the user inoptionally rotating the transmitting array into a known orientation tothe local magnetic field. Such specific positioning is not needed forrelative navigation but can be helpful if it is desirable to relate thedata collected during navigation to a world coordinate system.

On the receiver side, an additional active calibration or signalreference coil can be incorporated into the antenna node, or into thebody of the locator itself, to provide a self calibration capability.This calibration coil can be used to calibrate the relative sensitivityof each coil within the node. The active signal from this coil can alsobe used to allow the processor to determine the relative positions ofindividual antenna nodes with respect to each other and the coordinatesystem of the instrument. In one embodiment of our invention, thiscalibration coil is integrated into the interconnecting element alignedapproximately with the mating plane of the two hemispheres. Anotherembodiment of our invention uses mix ferrite, Permalloy, GMR, GMI, andair coils within a node. Another embodiment distributes air coils alongthe arms. Another distributes ferrite, Permalloy, GMR, GMI, and air coilsensors along the arms to achieve minimum interaction between the nodes,while still another embodiment locates conductive spikes in the leg tipsto do the magneto telluric inverse at the same time. Another embodimenthas isolated nodes and tripods that communicate on a local area networkssuch as 802.11 or Blue tooth and use central processing means and/ordistributed processing means.

Our multi-sensor mapping locator can be used to track a sonde associatedwith a pipe inspection camera. The locator is placed in a fixed positionwithin signal range of the sonde, and the sonde is moved and tracked,mapping the track of the pipe in three dimensions. Importantly, thepitch or slope of the pipe may be accurately measured without the needto place a pitch sensor in the sonde or camera.

Our mapping locator can be successively moved in leap frog fashion,along the path of the pipe being mapped to create a connected map largerthan the transmitting range of the sonde. Unlike prior art, only asingle locator is required to accurately map the path of the sonde.

One or more navigational beacons can be placed to allow mapping within awork area. One or more GPS receivers can be incorporated into ournavigational beacons to allow mapping information to be related to oneor more coordinate systems, one example being latitude, longitude. A GPSreceiver can also be incorporated into our mapping locator. If two ormore GPS receivers are part of the locating system, then DGPS techniquescan be employed to further improve mapping accuracy.

Radio links between the locator and our navigational beacons can beincorporated into one or more of the navigational beacons in use withina work area. Radio links between the mapping locator and any number ofnavigational beacons may form part of a wireless network.

Mapping data may be stored within any system component that is part ofthe locating system. A preferred approach is to store mapping datawithin the portable mapping locator. However, in certainimplementations, the mapping locator can act as a data transceiver andsend either raw magnetic sensor data or further processed data onwardvia wireless techniques to a data logging, or display, or controlcomponent, of the locating system, for example a portable computeroptionally configured to receive, store, display or further process saiddata. Stored mapping information can then be used to update a geographicinformation system (GIS). An over determined navigational system thatcan use Kalman filtering techniques to improve accuracy is alsodisclosed.

Our mapping locator can be placed on the ground directly over one of thenavigational beacons as part of a calibration process. In a non-contact,non-aligned manner, the locator can more accurately determine the tiltand orientation of the beacon using its own orientation sensors.Additional information can be exchanged between the locator and thebeacon during this process taking advantage of the known close proximity(IR optical communication for example). The locator can measure thesignal strength of this beacon very accurately, during this operation.

Acoustic sensors can be placed in association with the navigationalbeacons and means is provided via a radio or other data link to transmitthis data back to the mapping locator. This listening can be used todetect leaks in piping systems. Leaks in building foundation slabs, socalled slab leaks could readily be pinpointed using known correlationaltechniques. The advantage of this system over known systems is itsmapping capability. In another related embodiment, the mapping locatoris provided with a sound source to inject acoustic energy into theground at a known location. Tomographic imaging techniques can therebybe utilized to develop acoustic images of subsurface structures. Thelocations of the sound sources are known relative to the receivers. Aground penetrating radar (GPR) device can be incorporated into ournavigated mapping locator, SAR or tomography techniques can be employed.

Additional navigation sensors can be integrated into our mappinglocator. The addition of sensors allows navigation in the absence ofnavigation beacons or it can improve mapping accuracy by providing anover determined system or allow mapping excursions beyond the range ofthe navigational beacons. Navigation beacons can be incorporated intotransmitting devices designed to put tracing signals onto buriedutilities. The frequency of the beacon built into the line transmittervaries according to which utility channel had been selected by theoperator for tracing. A plurality of distinct utility channels can beused to facilitate utility identification during mapping.

Our multi-sensor mapping locator can be used to track a sonde optionallyassociated with a pipe inspection camera. The locator is placed in afixed position in signal range of the sonde and the sonde is moved andthe track of the pipe is mapped in three dimension. Importantly, thepitch or slope of the pipe may be accurately measured without the needto place a pitch sensor in the sonde or camera. In order to allowaccurate slope measurements to be made, the user can manually level thelocator using a bubble level or other leveling device. Alternatively, atwo or three axis tilt sensor can be incorporated into the locator toallow true slope measurements to be made without the need for levelingthe mapping locator. Any other signals due to cables and pipes in thissame area can also be simultaneously mapped by the methods described. Ina further improvement, the mapping locator alerts the operator with asignal to indicate whenever the sonde being tracked moves into or out oftracking range. Such an alert signal can be done, using any meansavailable, such as sound or light or vibration, discernable by thesystem operator. Such means might use a remote signal divide in nearproximity to the operator. The remote signal device might use a radiolink between the mapping locator and the remote signaling device. Theequations needed to make such positional calculations are known. See forexample, U.S. Pat. Nos. 4,054,881; 4,314,251; and 4,710,708, the entiredisclosures of which are hereby incorporated by reference.

Our mapping locator can be successively moved along the path of the pipebeing mapped to create a connected map larger than the transmittingrange of the sonde. In this method, before the sonde moves out of thedetection range of the mapping locator, the movement of the sonde isstopped and the mapping locator is moved to anew fixed position allowingfurther movement of the sonde within the detection range of the mappinglocator. The piping system is thereby mapped in segments, and eachsegment is mapped with the locator in a fixed position. Successive,connected segments are mapped from successive locator positions. In thismethod of operation, the locator can optionally signal the operator whenthe mapping locator needed to be moved to a new forward position (orrearward depending on the direction of sonde movement), to allow themapping locator to remain within mapping detection range.

In the absence of additional navigational sensors, the sonde remainsfixed and is not moved, while the mapping locator is being repositionedto a new forward location. In this basic method, using no othernavigational sensors, certain positional and orientation ambiguities canarise. If the axis of sonde being tracked is nearly vertical, then itbecomes difficult for the mapping locator to accurately resolve itsorientation with respect to the horizontal orientation of the pipetrack. During locator repositioning, if the sonde axis is not nearvertical, the locator can use the signal from the sonde, as anavigational beacon, to track its own position relative to thecoordinate system of the piping system being mapped. During this processthe mapping locator can guide the operator repositioning the instrumentto a desired new location based upon a predicted direction of sondetravel. During this process the locator can guide the operator in such amanner to stay within detection range and avoiding positions that mightresult in positional or orientational uncertainties of the mappinglocator with respect to the sonde. The addition of a compass allows themapping locator to determine its own orientation with respect to theworld coordinate system. In the simplest configuration, a mechanicalcompass might be employed and the operator instructed to always placethe orientation of the locator in specific orientation with respect tothe compass needle after each successive repositioning movement. Afurther improvement is to provide a means for the mapping locator tomeasure its own orientation with respect to the local fixed magneticfield (typically that of the search), for example, incorporating anelectronic compass into the instrument. If the orientation of the fieldsensed by the compass is constant at each position, the mapping locatoris moved to, then each mapped segment will have a correct orientation tothe previously mapped segment. Using a single axis sonde, an unambiguousdetermination of the relative position of the sonde and the four-antennanode, mapping locator, can be obtained if the orientation (yaw, pitch,roll) of the locator is known relative to the earth coordinate system. Acompass and a multi-axis tilt meter is needed in the locator to resolveuncertainties with respect to sonde signal strength, ground slope, sondeorientation or any need to remain with detection range of the sondeduring locator repositioning.

In another embodiment, one or more navigational beacons are placed toallow mapping of unknown underground objects, within a work area. In apreferred embodiment, these beacons are low frequency (approximately 1Hz-10,000 Hz), single or multiple axis, dipole transmitters. Lowerfrequencies tend to not inductively couple onto other objects within themapping work area. In choosing frequencies, a balance must be struckbetween minimal coupling and high levels of ambient low frequency noiseor jammers found in many environments. A simple mapping configurationutilizes a single axis dipole transmitter in combination with ourmapping locator. The mapping locator is configured with a two-axis tiltsensor and an orientation sensor such as an electronic compass. So longas the mapping locator is not inverted, a three-axis tilt sensor is notrequired. The tetrahedral, four position, 3-axis antenna nodeembodiment, of our mapping locator does not require that these singleaxis beacons be placed in any particular orientation. For someapplications, relative navigation within the work area is all that isneeded, while other applications require navigation relative to a worldcoordinate system. If more than one beacon was employed simultaneously,then known means are employed to make the signals distinguishable.Frequency coding, for example, is one simple means wherein each beacontransmits a signal at a specific predetermined Frequency. A known codingscheme can be employed to make the signal from each beacon separatelydistinguishable. Two and three axis beacons allow locator configurationswith fewer numbers of antennas or allow a more over determined systemand greater potential locate accuracy, especially in noisy environments.A single axis dipole transmitter can be rotated at a highly constantrate in various known techniques. The flux angles and phase reversals ofthe transmitted signal can be used to accurately determine an angularorientation measured around the transmitter's axis of rotation.

In one embodiment a crystal controlled servo or stepper motor can rotatea horizontally disposed, dipole transmitter around a vertical axis at ahighly constant rate known to the mapping locator. The rotational rate(frequency) must be small relative to the transmit frequency.

In areas where an accurate compass bearing cannot be measured, theforegoing technique provides information to allow a relative bearingbetween the beacon and the mapping locator to be established. Apractical way to make a low frequency magnetic dipole beacon is to spina permanent magnet around an axis approximately normal to theNorth-South pole axis of the magnet by known means, such as using abattery powered electric motor. Using vertically oriented beacons placedon a horizontal surface, measuring at constant height, field strength isa function of range only. The spread apart sensing arrangement of thepreferred embodiment of our mapping locator can utilize the 1/R³property of the vertically oriented dipole field to determine the rangeand relative bearing of the beacon from the mapping locator. However,unless the orientation of the locator is known in world coordinates,using a single axis vertical beacon, the locator can lie anywhere on acircle centered on the beacon. As stated elsewhere this ambiguity can beresolved by using a compass to establish the absolute orientation of thelocator. The limitation can also be resolved by using two or morebeacons without requiring the use of a compass. Unless the operator ishighly skilled at holding the mapping locator in an accurate verticalorientation, a two axis tilt sensor is highly desirable in the locatorto allow the locator to constantly correct for changes in tilt fromvertical. Standard known techniques for transforming one coordinatesystem in to another are known and can be employed to make thesecorrections.

All discussions herein about navigating our mapping locator with respectto one or more beacons assume that the beacons are within a range whereadequate signal levels are available to allow navigation to some levelof desired accuracy. Two beacons can be used to create a simplenavigational net or array. A line that runs from one beacon to the otherdefines a navigational baseline. So long as the mapping locator remainson one side of this baseline, unambiguous navigation can be achievedwithout requiring an orientation sensor in the locator. Adding a thirdbeacon in an approximately triangular configuration resolves thecrossing of the baseline ambiguity, and provides an over determinedsystem for greater locating accuracy. These beacon navigationaltechniques are similar to long baseline acoustic navigational methodsused in navigating vehicles underwater, and many of the same lessons andtechniques can be applied. For example, the positions of the beacons canbe completely unknown to the locator when placed initially, andwell-known, iterative error reducing techniques can be used to determineand refine the known positions of the beacons during the survey of thework site. For example, simply crossing a baseline between two beaconsallows the distance between the two beacons to be accurately determined,as the sum of the measured distances to both beacons will go through aminimum.

There is no real limitation to the number of beacons that might beemployed to survey and map an area. As a general strategy, it will bedesirable to fix the beacons in place while the locator is moving andvice versa. During beacon repositioning it will be desirable to set thelocator into a fixed position so that it can track the position of thebeacon during the repositioning process. Beacons can be color coded orotherwise marked to distinguish these as being unique. Beacons can allbe identical and incorporate a switching means to allow specifictransmission channels to be selected. Beacons of the same frequency canbe employed in larger navigational array so long as they were notdetectable by the mapping locator at the same time. One advantage ofusing a simple vertically oriented sonde is simplicity and cost. If thearea being mapped is relatively flat and horizontal, relative signalstrength alone measured with the sensing array of the mapping locatorcan be used to determine range and bearing of the beacon from themapping locator without requiring computations of flux slope and dipolefield models. A vertical sonde will also not induce a signal into aburied utility if it is placed directly above the utility.

In another embodiment, one or more GPS receivers are incorporated intoour navigational beacons to allow mapping information to be related toone or more coordinate systems, one example being latitude, longitude. AGPS receiver can also be incorporated into our mapping locator. If twoor more GPS receivers were part of the locating system, then DGPStechniques can be employed to further improve mapping accuracy.Incorporating GPS receivers into the navigational beacons is highlyadvantageous since they remain in a fixed location while the mappinglocator is moved throughout the area being mapped. In one embodiment,GPS data can be transmitted to the mapping locator continuously by wayof a radio or other means separate from the beacon signal itself. Inanother embodiment, the GPS data can be encoded onto the beacon signalitself by known means. Horizontal drilling sondes transmit pitch, rolland temperature and other data by various means.

In another embodiment, time stamped GPS data is stored in the beaconassembly for later transmission or downloading by wireless means orremovable media. This technique is highly advantageous in that simpleshort-range transmission techniques might be utilized to communicatethis data at the end of the mapping session just prior to recovering thebeacon. A simple low cost, low powered IR or radio link might beemployed between the mapping locator and the beacon to exchange thisinformation. If a GPS receiver is also incorporated into the mappinglocator the data can be time stamped and stored. This method allows forafter-the-fact DGPS processing of the position of the locator and thebeacons. A hybrid navigating system can be configured using thedescribed beacons where navigating the mapping locator from a GPSavailable area, into areas where GPS is not available. GPS reception canbe available for some beacons in the navigational array. For example,they can be positioned outdoors in areas of good satellite skyvisibility, but not available to others placed indoors inside abuilding.

In another embodiment, radio links between the locator and ournavigational beacons are incorporated into one or more of thenavigational beacons in use within a work area. Radio links between themapping locator and any number of navigational beacons can form part ofa wireless network. Further, any beacon or locator which does receiveGPS could re-transmit time code data to allow for simultaneous timestamp of other devices in the network which are not receiving, GPSsignals.

We have invented a method of measuring the movement of the antennas of alocator relative to the field(s) and using this information to map thefield in space. Preferred embodiments utilize acoustic Doppler, radarDoppler, optical (flow) navigation (both imaging and non-imaging),inertial navigation, E-Compass, tilt sensors, GPS, DGPS, sondenavigation, short baseline, and Kalman filtering techniques. Our newlocator has the ability to store and spatially determine positionsrelative to the coordinate system of the locator as well as to thegeographic coordinate system. Rolling or mechanical tilt (drag) sensorscan also be used for determining motion relative to the ground.

Acoustic measuring techniques can utilize one or more beams. At leastone means for determining the field flux vector is desirable. Kalmanfiltering can be used in this application in a GPR context.

Our locator takes advantage of a new antenna geometry that utilizesthree axis orthogonal sensor (GMR and similar) pairs approximatelysymmetric around a common center point. Coil pairs can be summed orsubtracted. Subtraction allows a field gradient along the coil pair axisto be established. This geometry allows a common center point, yetallows all of the coils to be identical and so to have a commonelectrical response. This is important for operation at a wide range offrequencies.

The coils can be placed on the faces of a cube. Three of the faces canhave low frequency response and three of the faces can have highfrequency response. The leakage inductance between the faces is whatdecouples high frequency and low frequency coils. There is norequirement for orthogonality because of improved processingcapabilities. The balance point for fields aligned with an intermediateaxis is useful for calibration.

In one embodiment, the first signal processing means is located INSIDEthe space bounded by the antenna sensors. In another embodiment,calibration coefficients are stored within the space bounded by theantenna sensors. Within the housing a set of three or more sensors canbe used. In another embodiment, acoustic or optical navigationaltransducers are mounted approximately at the intersection of one or moresensor axes and the outer surface of the receiving antenna enclosure. Instill another embodiment, the first signal processing means is mountedin a plane perpendicular to an axis intersecting the approximate centerof said antenna sensor arrangement where said axis has an equal anglerelationship to the three mutually orthogonal sensor axes. A supportingstructure, typically a hollow tube, can transect the antenna arrangementor it might terminate and affix to the supporting enclosure. The sensorarray can also incorporate additional sensors such as a compass or a twoor three axis accelerometer. Another embodiment has the sensor arrayincorporating an internal calibration coil. If the geometry is solid,the best calibration scheme involves placing a single turn on eachreceiving coil and calibrating the channel gain. The single turn is verycontrollable and very broadband when in series with a large resistor. Ifthe channel gains are solid but the geometry is sloppy, the bestcalibration scheme involves a set of two or more coils to make knownfields. A combination of these two calibration schemes can be utilized.A combination with calibration coils halfway between can also provideuseful results. The more independent measurements that are available,the better the calibration.

The calibration signal can be shifted in frequency or amplitude from thetarget signal. Linear interpolation can be used between discretecalibration frequencies. The calibration signal can be PRN or truerandom white noise so that calibration can be obtained at the exactoperating frequency by cross spectral density techniques.

In another embodiment the sensor array incorporates an internalcalibration coil in the plane of the first signal processing means asdescribed above or in another plane orthogonal to said plane. Stillanother embodiment has the sensor array incorporating an internalcalibration coil which can transmit a calibration signal. Thecalibration signal might be intermittently or contiguously transmittedduring locator operation. The calibration signal might be slightlyoffset from the target mix down frequency. The calibration signal mightvary in intensity as some function of received signal strength. Inanother embodiment the field sensors can be flat planar coils of wire.In another embodiment the field sensors would be flat planar coils ofwire consisting of separate inner and out coils with substantiallydifferent resonate frequencies allowing a greater range of signalfrequencies to be accurately sensed. In another embodiment the fieldsensors can be wound on cores and affixed on each sensor axis.Processing means can be provided within the enclosed volume to convertsensor signals into a digital form for transmission outside of saidenclosed volume. This allows analog transmission schemes.

Because the first signal processing means is approximately centeredinside of said sensor array, certain interfering signals can be knownand thereby subtracted or otherwise filtered from the desired targetsignals received. Digital noise cancellation techniques can be utilized.It is possible to measure the currents from the power supply and/or usevery small loop antennas very close to processing means.

The sensors can be connectorized and not hard wired. Cable and connectorconnection from remote pods back to a central processing means can beadvantageous. Cable and connector connection between distributedprocessing means can also be advantageous. In another embodiment, anactive transmitting field generator is incorporated (marker balls and/ormetal detection).

Multipoint techniques can be utilized whereby the sensors can bearbitrarily placed in space and need not be clustered in groups normutually orthogonal. Our tripod locator places a multi-point array ofsensors in space. It is compact and foldable. It is also self-standingfor fixed deployment. A folding baby crib (rectangular parallelepiped)with sensors distributed along the edge can outperform the tripod. Thetripod gives very good lateral location but directional drilling needsvery good vertical location. The drill needs to go over or under ahorizontally extended target somewhere. Our locator needs to havesubstantial vertical extent. A four-legged tripod with a mast upwardswill outperform currently available commercial locators. Taking noiseinto account, an inverted tripod array can perform better. It placesmore sensors where the noise is greater for more averaging. The sensorsin our locator can be cable connected or wireless connected. Ethernet,Bluetooth, std RF, IR, GPS can be utilized in each node. A navigationsystem can be used to place these and spatially relate the remotesensors to an earth-based coordinate stem.

Our tripod locator geometry does not require a precisely known geometry.It is possible to drive the remote node coils active (sort oftransponders) to allow the “net” to self-calibrate the positions of allof the nodes with respect to each other. One method is to cyclecoil-to-coil in a fixed sequence and apply the techniques as taught byU.S. Pat. No. 4,314,251 of Raab (incorporated herein by reference) tolocate each node with respect to its neighbors. A two axis accelerometercan be mounted in each node for precisely measuring tilt. Means areprovided to transmit other sensor data from each node. Our invention canutilize a probe insertable into the ground such as a waterjet. A vacuumexcavator nozzle with sense coils can also be constructed.

An anti-collision system for utility avoidance can be provided by firstlighting up buried lines, then sensing bore head sonde-induced fields inthese lines (HF, line illuminator added feature), thus providing awarning system upon close approach of a drilling sonde. The sametechniques can be used for pot holing and digging. Super low frequencysignals such as spinning magnets on the drill string can be used fortracking. Passive sonde techniques include GMR, GMI sensors ande-Compass technology. Direct calculation methods or direct positionalsolution methods can be used. The prior art has relied on specialcharacteristics of the dipole field, i.e., the so-called locator point,to determine the boring had position and orientation. Our techniqueallows a closed form solution on the position of the drilling headrelative to the locator and by inference with respect to an targetboring path. Continuous, real time monitoring and control of the boringprocess is possible. A single LONG sonde allows tilt to be measureddirectly by the system. A leapfrog drilling technique is also possible.Raw node data can be transmitted at a low rate to a control systemlocated where the drilling operator is located. Each remote node canhave full process means using dipole technology and some or all nodescalculate and display an RMS or Maximum Likelihood solution as well.Calculation can be based on the field shape of the drill string notbeing a dipole. Automated current inspection of drill sections can bepart of a drill string rack. Computer-assisted placement of nodes ispossible. A computer can be provided with a utility map. The computercan calculate where a sensor needs to go to reduce ambiguity. It is alsopossible to locate a tilted sonde in the tip. It is possible to tell ifa rotating tilted sonde is approaching a utility on axis of rotation ornot by monitoring the current in the utility with a current probe. Ifyou also have rotation angle you can tell if the drill is approachedabove or below. An inverse solution calculates the inclination of theaxis of rotation directly. A voting scheme can be used. A Kalman filterthat uses both the tilt sensor and the inverse solution is possible. Atilted sonde can be placed in the drill since it is essential NOT to hitthe utility. In the case of vacuum pot holing it is essential TO HIT theutility, so having the display on the side of the nozzle isadvantageous. The worker wrestling the nozzle has the display in frontof him. Placing four sondes in a nozzle allow you to get range andbearing. A tall pole version of our locator can be designed fordirectional drilling and have a jack for a nearby accessory antenna.

An improved mapping locator can utilize an array of flux antennas usingmulti-point field sampling in said locator to determine the location ofobjects of interest. Advances in numerical computing power allow thecost effective use of larger numbers of antenna coils than prior artcable locators.

A preferred embodiment of this locator utilizes a single verticalantenna mast with a lower and an upper antenna as well as two additionalantennas preferably on hinged supports that can be positioned to form atetrahedron with sides of approximately equal length. In this preferredembodiment, each antenna contains three or more mutually orthogonalcoils. Optionally each coil can be split into a coil pair sharing anaxis of symmetry.

Using multi-point field sampling, it is possible to determine thedistance and direction to an extended line source, such as a cable orpipe, using the ideal signal spreading relationship 1/R. Similarly, thedistance and depth to a compact dipole transmitter can be determined.

This technique has the advantage over currently available locators, ofbeing able to directly locate the range and bearing of dipoletransmitters placed beneath a reference surface, such as the ground,without the need to determine the location of a special place in thefield where the field lines are either vertical or horizontal. The depthof the dipole transmitter below the reference surface can also bedetermined.

We will call the point on the ground directly above a subsurface dipoletransmitter a “Sonde Locate Point.” In the case of the antenna array inthe preferred embodiment noted above, this sonde locate point will occurvery near the centroid of the three point base of this tetrahedronarray. This dipole locating method has the advantage of allowing theoperator to traverse nearly directly to Sonde Locator Point withoutfirst locating points in the horizontal plane where the magnetic fluxvector is vertical. A search pattern is not needed. The sonde LocatePoint is indicated on the display in the direction of the highest fieldintensity, independent of the orientation of the magnetic field lines.

The accuracy of a dipole locate can be improved by assuming a dipolemodel of the source and correcting for the variation in dipole fieldstrength with respect to the axis of the dipole. The sonde Locate Pointis preferably displayed relative to the coordinate system of the locatorand will move dynamically on the display proportional to the relativemotion between the locator and this point. This antenna configurationallows the direct tilt of the dipole transmitter to be measured. A keyadvantage of this improved locator in terms of cable and pipe locatingis that the depth can be rendered even when the locator is not directlyabove the line. Both field intensity as well as field gradientinformation can be used to calculate the displayed position of thiscable. Many typical locates contain multiple pipes or cables at variousdepths and orientations. Using both field gradients, as well as fluxvectors at multiple locations provides an improved ability to displaymultiple utilities or distorted fields.

Utilities often cross at different depths, occur in the same trench inparallel and also at different depths and also intersect in Tee's. Thebroader sensor footprint allows for significantly improved displayrendering of these complex situations. In one embodiment, the antennacoils are aligned with respect to the antenna supports. The antennaarray need not be tetrahedral in shape, but can be any arbitrary shapeprovided the coil positions can be determined. Each antenna can beseparately calibrated. Such calibration is facilitated from amanufacturing point of view if the calibration data is locally storedwithin this antenna housing. It is further advantageous, but by no meansrequired if the magnetic flux signals sensed by the coils are convertedto digital form within said antenna housing. The positions of the hingedarms could be determined by position sensors or simply by processing thefield data to determine if the hinged supports were open in a spreadposition or closed in a more compact configuration.

While the numbers and types of the antenna supporting structure can beof almost any form, the three legged tripod structure shown in FIGS. 1-3has the advantage of being stably deployable on uneven surfaces whileallowing the operator to conveniently view the display screen. Theforegoing techniques can be combined with other sensors including GPS,DGPS, tilt, accelerometers, compass, magnetometers, etc. The foregoingtechniques can be combined with a marker excitation coil. This antennaarray supports the capability to make depth measurements of makers.Individual antenna housings can be configured with more than threecoils. A preferred embodiment of three pairs of coils allows thedetermination of field gradients between coil pairs. Dual trace, currentdirection, RF, tuning, dual channel mixers, optional mix down to DC, lowsampling rates, many filters, the ability to fine auto-tune due tocrystal drift and optional temperature compensation are usefultechniques with our locator. A color display may also be useful.

Optical navigation represents a means to use gradient locating with asingle antenna. A memory is advantageous for this function. Further, ameans to enable a mapping function is possible. Mapping data can bestored in a solid state, hard disk or other memory, or similar media.Said media could be fixed or removable. For fixed media it is necessaryto provide a means to download data, by wired or wireless or optical orother remote data transfer means.

If only gradient locating is being employed without mapping, then onlythe direction and rotation of the movement of the locator over theground is needed for directional sensing. If accurate mapping is neededthen a true velocity is needed and the height of the optical sensorabove the reference surface, or ground, is needed. Optical mouse IC'scan be used. Ultrasonic height reference, laser spot(s), pattern andacoustic Doppler navigation can be used.

For multiple locator configurations, it is possible to integrate a radiolink to send data to a central point. A DC in jack is advantageous forthis type of usage. A sonde can be added to each unit to allow each tolocate its neighbor (directional drilling applications).

Our locator can have the ability to show a CURVED line (or band) on thedisplay. Only an array of sensors can do this in a meaningful fashion(or else of course a navigated sensor). A line from each of the fourantenna nodes can be indicated on a display. The field data from eachantenna node can be transformed into the coordinate system of theinstrument or the world (relative to each normal). A color or gray scaleor patterning (dotted-dashed line upper ball) can be used to distinguishthe lines. The locator assumes a cylindrical field for each, andcalculates an apparent distance from each ball using the informationfrom the other three to determine a relatively scaled offset for eachline on the display. If the line was straight and the return current wasuniform (no skin effects or return current effects, then all four lineswould align on top of each other. Variations from the ideal case can beinformation rich.

The three lower antenna node lines can be used as boundary edges tocreate an area to display. The edges of this area can be smoothed usingcurve fitting, i.e., utilizing a three or four point spline) and then agray scale contour can be used to display this are by various means.Narrow areas with approximately parallel opposite edges would be darkerand more distinct, broad trapezoidal areas would be less dark andfeather to indistinct edges.

The information from the lower triangular array can be used to deriveand display a single line that may or may not be curved. A variantinvolves averaging the angular orientation of the three nodes to apredicted line orientation and using this to set the orientation of theline at its midpoint. Similarly, it is possible to calculateorientations for the line at the two ends where they would intersect theedges of the display (some offset distance from the apparent linemidpoint). The midpoint of the line would be offset from the displaycenter point (instrument origin) proportional to the slope of the fielduse this simple approach to draw an arc with some radius of curvature. Avery large radius of curvature would be displayed as a straight line.This should have a similar effect to that just explained. The overlaidlines can be translocated into a gray scale pattern to indicate anapproximate flux pattern. In the ideal, cylindrical field case, thiswould look very much like a line.

The locator can assume that the field is due to a single conductor withsome radius of curvature. It can further make the simplifying assumptionthat this single conductor lies within a plane. Using the top ball thelocator can get a conductor plane tilt. A best fit solution using fielddata from all of the coils can be used to calculate the position,orientation and radius of curvature of this conductor model. This curvesolution can be displayed, projected onto the plane of the display.Curves can be displayed as ellipses if the plane of the conductor andthe plane of the display are not parallel. The locator can determine howwell the solution matches the model. If the match is perfect, thelocator can display a sharp and distinct line. If the simple modelsolution matches the “single conductor arc” model poorly it can thendisplay the “line” as the gray scale band following the path of theline. As the match degrades due to complex fields which do not match asingle conductor model, the locator can make the gray scale line widerand lighter in color with less distinct edges.

The use of a color display allows the locator to encode depth, tilt etc.with color, for example, red for close, blue for far (Z depth, not rangefrom the instrument). Gray scale display along the lines can be added.

It is possible to create a “laser light show” on the ground of where theutilities are. A pair of motorized mirror faces and a laser can be usedto do this. Since you know the height from the ground, it is possible toproject, using vector graphics to drive the laser, a simple indicationof the pipes, cables and other objects located underground. Since thisfeature of the locator shoots “down” and would have a safety interlockso that it would not fire unless pointing down and level, the potentialfor eye damage is minimized. Also, preferably the locator has a triggerthat must be squeezed or held to make the locator project images of thelocated utilities. Projecting these images on the ground is the mostintuitive way of displaying the information. The need for a display onthe locator itself could be eliminated. One or more vibrating pagermotors could be mounted in the handle of the locator to indicatedimension of information.

Communications between a locator and transmitter could be handledthrough a “router” device. A laptop or PDA would be optional for this,since it could be a central orchestration point for all devices, andprovides an easy access point for the data, massive storage, easy way toedit text and other site information, a standard way to include othermedia (digital pictures of site, etc.). This architecture also removesthe burden of storage from the locator, and places it in the laptop-PDA,thus simplifying the architecture, etc. This makes it much easier toaccess and provide integration to GIS databases, Internet, etc. A PDA issomething that one can carry around whereas a laptop could perhaps beleft in the truck logging as you go. It may be desirable to provide adirect device to device communication for triangulation. Communicationsprotocols such as Bluetooth and 802.11b (and others) are now availablein relatively inexpensive modules, and built into many laptop/PDAdevices as standard.

The number of sources and the number of unknown variables to be solvedin an inverse problem must be provided to most algorithms. Utility mapsprovide a reasonable estimate of the number of unknowns to be solved.The number cannot be regarded as exact because there may be abandonedutilities in the survey area that predate available maps. The presenceof abandoned utilities would lead to an underestimate of the numbers ofunknowns. It is also possible that a known utility is severed at one ormore locations with a resultant lack of emission and an overestimate ofthe number of variables to be solved. Despite these limitations,existing utility maps do provide a usable estimate. The personalexperience base of operating personnel can provide additionalinformation on the number of unknowns to be solved for and provision foruser input could be made.

Optimal (Maximum likelihood, LMS, maximum entrophy) solutions of thecomplete problem require that the solution of the navigation problem beintegrated with the solution of the magnetic source inverse problem.Measurement noise exists in both the position measurements and in themagnetic and electric field measurements. Unless these error sources aretreated together, unnecessary error will be introduced into thesolution. Prior art solutions have added commercial navigation systemsto existing utility locators.

Gradient strategies have limitations for several reasons. Theinterfering fields in any small volume are strongly correlated andtherefore, are unlikely to behave like noise, and they will not averageout (EMC). Additionally, local variations in permeability andconductivity can distort the field sensed by each of the coils, therebyresulting in location errors relative to the earth surface coordinateframe, like GPS (Field Distortion). The data from multi-point fieldsensor systems can be combined in many ways, such as by utilizingmaximum likelihood, voting schemes, fuzzy logic, minimum entropy, MUSIC(Multiple Signal classification), SAM (Synthetic Aperture Magnetometry),least squares of independently estimated positions, and Kalmannavigation filtering. The goal of the Kalman navigation filter is to mapthe path of the sewer pipe or other target utility, by incorporating allavailable information, such as length of push rod fed, model of camerapush rod stiffness, etc.

As a drill string approaches the utility, it can induce a signal in theutility that can be detected by a current probe on the utility andverify that the drill string is either clearly above or below theutility. This information needs to be consistent with the multi-pointfield monitoring.

Many states now require, by law, potholing to expose any utility beingcrossed and a person stationed at the pothole.

An antenna array on the nozzle of a vacuum excavator provides theability to place sensors below the surface of the ground. Thecombination of below ground, surface and above surface sensors isunique. The combination of fixed and moving sensors is unique anduseful—it improves the resolution of drift in source strength versusmotions of the receiver and source. In a typical vacuum excavator nozzleapplication, the coils would be in a replaceable nose piece. Theprocessing electronics may include inclinometers, accelerometers, ortilt sensors so that if the excavation proceeds at some angle to thevertical, the position of the nose pieces may be calculated relative toa point above the surface being excavated. The processing electronicsmay include position sensors such as GPR/WAAS, range sensors such as asonar, radar, or lidar to measure depth of penetration.

A short section of pipe connected to a pressurized water source is acommonly used low cost method of drilling short distances into dirt andtunneling under sidewalks and driveways. It has the distinct advantageover mechanical drilling in that the operator can sensitively feelutilities and stop advancing the tool. Equipped with a low lost set ofcoils that can be plugged into separate processing electronics, itbecomes a very effective method of emplacing sensors at a buriedutility, either on land or underwater, to very precisely locate theutility. The operator of the drill may then deviate the course of thedrill or abandon the hole as needed. The waterjet emplaced coils mayalso be connected to electronics that automatically control the drill todeviate it around the utility.

Helicopter and aircraft mounted geophysical survey instruments arecommercially available. Geophysical survey instruments generally treatpipeline and utility signals as interference. Current horizontaldirectional drilling operations typically use two operators, one tocontrol the drill and one to move the locator. A typical task requiresdrilling under a freeway. A sensor set that can hover over traffic orland in the median provides an obvious advantage to the operatingpersonnel, avoids having to shut down traffic, and at the same timeallows a single operator to accomplish both tasks.

In some applications such as hazardous material dumps and minefields itmay be difficult or hazardous for personnel to deploy sensors. In suchsituations, the sensors may be deployed by manned or remote controlled,fixed wing or rotary aircraft. The sensor may be used in flight orsimply deployed on the ground. The sensor may be disposable or retrievedat a later time.

The airborne sensor may include a radar or lidar altimeter forcalculation of burial depth from the measured range to sonde or utility.The airborne sensor may include GPS/WAAS, Doppler, inertial, and/orcorrelation navigation systems to allow multiple range bearingcalculations to be made with a single sensor. Navigation capability alsoallows the preparation of accurate as built plans and the automatedstorage of plans in a Geographical Information System.

Oil and gas pipelines, telephone cables, fiber optic communicationcables, power transmission lines, and mooring cables often coexist onand within congested river, lake, harbor, and ocean bottoms.Maintenance, repair or addition of new utilities in such situationsrequires the location of existing utilities during planning, operations,and final documentation (preparation of as-built) phases. Fixed sensorsand sources may be attached to existing utilities. These sensors andsources can be powered by and communicate using the existing utility.These sensors may have independent power and/or communication and datastorage capabilities. These sensors may communicate with moveable and/ormoving sensors to provide optimal estimates of the absolute and relativelocations of existing and new utilities in real time. The moving sensorsmay be deployed by submarine, ROV, diver or AUV. Sensors may communicateby hardwire, electromagnetic coupling, sonar or laser. Underwatercommunications lasers are usually green to take advantage of higheroptical transparency in the green band. Underwater electromagneticcommunications are usually in the ELF band at larger ranges, butfrequencies above a megahertz may be used at very short ranges.

Such optical estimates may be derived by minimum least squares, normminimization where the norm can be Sobolev, Kalman filtering, maximumlikelihood or maximum entropy, Nelder Mead simplex search, or steepestdescent search methods. In the complete solution, all measures of range,position, amplitude, phase, velocity, acceleration, tilt and frequencyshould be regarded as noisy data and subject to revision to produce asolution with minimum error over all parameters. In some cases,particularly with battery powered, hand held or autonomous vehicles, thefull solution may be too computationally intensive. In such cases,optimization over subsets of the dataset may proceed independently toproduce partial optimization. In particular, the navigation data may beoptimized independently of the relative location data. Such computationmay be performed by a single central processor or distributed over anetwork of processors.

A common problem faced in locating is the burial of multiple utilitiesin a single trench. This situation can be particularly confusing whenthe utilities have direct connections at multiple locations. Oftenlittle knowledge about where the connections are made is available.Prior art locating equipment has assumed that a single utility or sondeis producing the magnetic field. The inclusion of the ability tocalculate distance traveled along some principally horizontal axis of alocator device can help interpret these situations. As long as theoperator constrains the motion of the locator to be only puretranslations along a single axis, useful information for solving theinverse problem for multiple sources may be gathered.

In the simplest version, measurements of a single magnetic fieldcomponent in the locator device's local coordinate system would bemeasured at a series of points as the locator is moved. A numericalvalue computed from these measurements would be displayed on a displaydevice as a function of position for interpretation by the user. Theposition can be measured by double integration from accelerations, bysingle integration from velocity, by time interpolation between fixedpoints, or in general by any of the navigation methods discussed in thenavigation section of this disclosure. These values can also be sortedfor later processing or transmitted. Navigational repeatability may bechecked by placing way points on the ground and returning to the waypoints by performing a precise reversal of the original route. Analternate means to check the navigational accuracy is to stop at a waypoint and rotate the locator by 180° on a line perpendicular to theoriginal navigation path. The processing means is then set to a closuretest mode. In closure test mode, the processor checks that the originallocation of a way point is substantially the same as the currentposition. Closure may be checked by substracting distances calculatedfor accelerations or speeds from the accumulated distance. If the singleaxis of multiple frequency magnetic fields be measured, is tilted fromthe vertical, earth referenced components can be computed atnavigational track crossings. A way point mode may be provided whereforward motion is stopped and the locator is rotated at a fixed point inspace to collect over determined data on the orientation of the multiplefrequency magnetic field components. This over determined data may beused to both calibrate navigation devices, measurement devices, andsimultaneously to calculate a least squares—best fit position, or moregenerally, a minimum norm position for several sondes and utilities.

A more capable version of the locator is to measure three components ofthe magnetic field in the locator coordinate system at each of aplurality of frequencies. Again, function of each of these componentscan be displayed versus distance traveled for interpretation by theuser, transmitted for remote processing, or stored for later reference.In particular, it is useful to calculate and display total fieldstrength, a horizontal component of the field strength, a verticalcomponent of field strength, and elevation angle of the magnetic field,and the azimuthal orientation of the field strength at each of aplurality of frequencies. Such search coordinate referenced componentsmay be trivially calculated from the instrument coordinate system,measurements with data from inclinometers, tilt sensors, oraccelerometers residing within the locator.

A still more capable version of the single navigational sensor locatoris to measure the components of the multi-frequency magnetic field at aplurality of locations in the locator. Mechanically, the simplest sucharrangement is to provide an array of sensors along a natural verticalaxis of the locator instrument so that as the instrument is moved alongthe sensitive axis of the navigational sensor, each magnetic fieldsensor makes multiple measurements along separated lines in space.Sensors with various sensing axes may be co-located at nodes along thevertical axis or distributed along the vertical axis or used incombinations of co-located and distributed sensors. In some mechanicalpackages it may be convenient to densely pack a number of sensors. Inthat case, it may be advantageous to measure the gradient components ofthe magnet field.

Means may be provided to the user to select a subset of the frequencies(or codes in the case of code division multiplexing or time slots or acombination thereof) to reduce the confusion on the screen. Means may beprovided to select particular combinations of field components at eachof the selected frequencies which are displayed versus distance. Thedistance axis may be arbitrarily scaled or warped. For example it may beuseful to logarithmically compress or exponentially expand the distanceaxis when displaying some functions of the field to reveal particulardetails.

The distance from a sonde in a horizontal directional drill to theutility that is closest in the direction of travel may be displayednumerically and/or schematically in real time on electronicallycontrolled display media such as a LED array, plasma panel, CRT, LCD, orelectronic paper. The measured tilt of the drill string, the depth ofthe drill string, and the depth of each utility may be displayed in apaged format on a single display or multiple display devices may beused.

Another useful display mode is to project the path of the drill into theplane of the utility and display the vertical separation that would beachieved with the current tilt of the drill string.

Showing schematically the projections of all utilities surveyed in aparticular area onto the plan perpendicular to the drill head sonde isanother useful display mode. This mode is useful for visualizing at aglance whether the current drill path will miss all known utilities in aparticular area. Means may be provided to download or enter coordinatesof existing or planned utilities, mines, well, aquifers, dykes, sills,or general geological features manually or electronically from adatabase. Downloads may be accomplished via Bluetooth or other IEEE802.11 (WiFi), IEEE 802.15 (ZigBee), IrDA or other wireless linkformats, IEEE 802.4 (Ethernet), USB, RS-232 or other wired formats, ordigital memory. Digital memory in the form of SDI, Compact Flash, andSmart Card are particularly convenient. A modification of this method isto show a selected subset of the measured, existing database, andplanned utilities in an area projected onto the plan perpendicular tothe drill head sonde. The subset may be selected by displaying onlythose utilities within a specified range, by displaying only the closestutilities up to some maximum number. The subset may be select by type:gas, water, electric, sewer, cable TV, telephone, fiber optic, reclaimedwater, or unidentified.

If more than one drill is being operated in an area simultaneously theprojected tracks of several drills may be shown projected onto the planeperpendicular to any particular drill. This display mode is particularlyuseful if the drill holes must be in some particular orientationrelative to each other. In the case of drill holes that are made tocollect gasses, leachates or seepage from a landfill or hazardous wastedisposal facility, it may be advantageous to have a number of smallercollector lines intersect and connect with a large central main line. Inthe case of injection wells, maintaining separation between the linesmay be more important so that the lines can be separately pressurizedand monitored for flow. Such lines may be left unlined, selectivelylined, or selectively lined with semipermeable and impermeablematerials.

Having a single, principally horizontal navigation axis, while capableof novel tasks, imposes severe constrains on the user. Incorporation ofadditional sensors on each axis of motion and processing all of thesensors through a Kalman Filter will improve both the ease of use andthe performance of the system.

Simply having a yaw axis sensor plus single axis distance measurementprovides an enormous increase in the utility of the instrument. The useris no longer restricted to walking in straight lines for properprocessing of the data. The user may instead walk in a circle or a FIG.8. Navigational repeatability may be checked by placing waypoints on theground and returning to the waypoint by a variety of routes. Oneembodiment is a locator with an elongate vertical axis and upper andlower antenna nodes. A pair of monostatic Doppler transceivers may bemounted at the approximate midpoint between the antenna nodes tominimize EMI. Alternatively, the Doppler sensors may be mounted at thelower end of the mapping locator to minimize the amount of transmitpower needed and to maximize the Doppler frequency shift by using beamangle close to grazing incidence. A coriolis force gyro or a MEMSangular rate sensor may be placed anywhere along the elongate axis.

There exist on the market a number of nail, stud, and wiring locators.None of the devices are suitable for producing a map of the interior ofthe wall such as might be useful to architects and builders whenplanning remodeling and additional. In this embodiment, a mappinglocator includes two ultrasonic or optical distance measuring deviceswith approximately perpendicular axes, differential capacitance sensorsfor studs, electrical antennas for live wires, and magnetic sensors forcurrent carrying utilities and camera sondes. In normal use on a wall,one of the distance measuring sensors will measure the distance to awall and one distance sensor will measure the distance to the floor orto the ceiling. A two axis accelerometer can be used to correct for theorientation of the mapping locator on the wall.

A particularly advantageous connection of transmitter to the plumbingsystem is to connect to the hot and cold water pipes at the outlet ofthe hot water heater. The dielectric isolator bushings in the hot waterheater will keep most of the current in the building. If the transmitterwaveform is asymmetrical in time, cold and hot water pipes can bedistinguished inside walls by the direction of current flow relative todirection to the hot water heater. Alternatively, the transmitter andthe mapping locator may be phase locked via an 801.11 link.

Another useful display projection is to display the measurement dataoverlaid on a photograph of a wall. In remodeling and renovation,drilling through wooden joists, forelocks, sills, and studs to pull newwiring is a common practice. Avoiding existing utilities is a well knownproblem. Addition of a small permanent magnet to existing flexible shankdrills would allow the current system to be used. As long as the drillis spinning, the time varying magnetic field from the spinning magnetcan be detected by the coils and processed to provide a locationestimate of the drill. With the incorporation of static magnetic fieldsensors such as Hall effect, fluxgate, GMR, or GMI sensors, the drillposition can be estimated with or without spinning. The acoustic oroptical distance measuring sensors may be supplemented or replaced withencoder wheels or optical motion trackers. The data from the wallmapping locator may be continuously transmitted to t a portable computerfor real time display or logged on solid state media such as SDI,Compact Flash, or Smart Card.

It is common practice in telephone and cable TV operations tooccasionally fold cables back on themselves and incorporate a length ofslack into a long run for future installation of a pedestal. Industrypractice is to place a buried marker at slack loops to make them easierto find. However, not all slack loops have a marker and not all locatorshave a marker detection feature. Thus, there is a need for automaticdetection of slack loops in a walkover locator. The physics behind suchdetection is relatively straight forward. If a current is induced in thecable, then over most of the length of the cable the magnetic field willbe approximately cylindrical. Where a slack loop is incorporated, therewill be a magnetic approximately dipole field at each end of the slackloop that is synchronous with and superimposed on the cylindrical field.Usually the magnetic dipole field associated is vertical, as the slackis buried with the turn around loops horizontal. Occasionally, the loopswill be tilted or even vertical when a large bend radius cable slackloop is buried in a narrow trench. An alternate method of creating aslack loop in a utility is to bury a multi-turn coil. In that case onlya single approximately dipole field will be associated with the slackloop.

While walking along the length of a buried cable run, the magnetic fieldwill normally be horizontal and perpendicular to the direction oftravel. Furthermore, the gradient of the magnitude will be directedstraight down. As the dipoles associated with the slack loops areapproached, the components of both the magnetic field and the gradientof the magnitude are distorted compared with the idealized cylindricalfield. One method of recognizing these dipoles is to store the depth andmeasured tracing current of the utility in memory. This depth andcurrent may be measured by the locator in an area away from the slackloop and stored into memory by a sequence of operator key presses.

On some utilities, such as cast iron sewer pipes, there is significantleakage of trace current into the ground even at low frequencies. Thisis particularly the case when there is no ground connection at pointsaway from the transmitter along the utility and most of the currentflowing is due to resistive or capacitive coupling off the surface ofthe utility to the earth. On long cables and at higher frequencies (400kHz), significant capacitive currents flow through the jacket and themagnetic fields are better approximated by a conical field due to alinearly decreasing electric current. In the case of significant leakagealong the length of a cable, a better model to use for the source fieldis a linearly decaying current along the length of the utility and anapproximately conical shape for the resulting magnetic field. In suchcases, better performance will be had by adjusting the nominal tracecurrent as a function of time based on at least squares fit of a subsetof measurement data to a magnetic field model of a linearly varyingcurrent source plus a dipole source plus return current in the earth.

Similarly, the depth of a utility may vary in a systematic fashion alongits length. The utility may be a free flow pipe with a designed grade ofa few degrees under level ground. The utility may be buried at anapproximately constant depth under sloping or rolling terrain.

The depth may be entered from “as built” maps. Without navigation, abest fit of the output voltages from the coils can be made to a utilitywith known depth and electrical current. These distortions can berecognized by having more than three sensor coils within each node or bycombining navigation information with the outputs of three or fewercoils.

The simplest locating situations are where there is a single sonde(dipole source0 or a single horizontal utility (electric current linesource or cylindrical magnetic field). These fields have fewer than sixfree parameters. Other utilities such as cast iron pipes, ductile ironpipes, concentric neutral power lines, concentric neutral power lineswith conductive polyethylene jackets will have fields that are bettermodeled as conical magnetic fields due to a linearly decreasing currentimmersed in a fairly uniform return flow. In highly conductive soils,the return flow may be come quite concentrated due to skin effect,especially at higher frequencies (100 kHz and up). If the walk overlocator is sampling or continuously measuring approximations tocomponents of the field, derivatives of the magnetic field, and/orintegrals over the field so that more than six parameters are known,consistency with combinations of simple field models, can be checked. Ifthe deviation from consistency exceeds some preset, or a user adjustablethreshold is exceeded, an alarm indication may be made. In addition, aset of alternative models, such as sheath fault, slack loop, Tconnection, Elbow, two sub-parallel lines, three sub-parallel lines andcombinations thereof may be tested for fit against the measured data.Test methods may be parameter fit, cross correlation, fuzzy logic,neural net, wavelet transform matching or similar means. What is unique,is the ability to automatically detect and/or display the type of modelthat fits the data best and to provide confidence bounds for the fit.

In one embodiment, the estimates of magnetic field vector and themagnetic field Jacobian may be measured at the top, middle, and bottomof an elongated axis of a locator instrument. The field itself isspecified by three independent components, and the Jacobian has nineindependent components so such a measurement constitutes 3×12=36independent measurements at a single point and orientation in space.This is sufficient for robust discrimination between several locatingscenarios. If, in addition, the locator has a navigation system, then 36independent measurements of the magnetic field can be made at aplurality of locations.

Further improvement in the performance of such arrays of sensor coilsmay be had by measuring and/or calculating the interactions betweencoils and the failure of each coil to behave as a cosine response pointreceiver. Such calibration may be performed by rotating the array ofcoils in a very uniform field such as that produced by a large diameterHelmholtz coil. Alternatively, the coil interaction may be calculated byfinite element, boundary element, or ATDT methods. A third method tocalibrate the array of sensor coils is to provide a calibration framesupporting multiple small coil magnetic dipole sources in a knownconfiguration. Each of the supported coils may be driven with aplurality of signals while the locator is moved near and/or within thecalibration frame. This produces an over determined set of data fromwhich the locator coil position, orientation, and frequency response maybe determined by least squares fit, simulated annealing, simplex, orsimilar methods.

For best navigation results, the energy emitted by the navigationsolenoids should be low frequency to avoid inducing significant currentonto utilities within the markout or survey area. As a generalguideline, frequencies lower than 10 kHz are appropriate for dipolesource navigation. The same solenoid may be simultaneously operated atone or more higher frequencies to induce current into conductors buriedwithin the earth or hidden within a wall. As a general guideline,signals with frequency content higher than 10 kHz are suitable forcoupling onto elongate conductors. Little current is induced in elongateconductors that are approximately within a plane containing the dipoleaxis. Detection of such conductors can be enhanced by providing one ortwo additional, mutually orthogonal dipoles at several places in themarkout area. The relative strength of coupling of the source dipolemagnetic fields provides an estimate of the azimuth and elevation anglesof the nearby conductors. The one or two additional dipoles also improvethe navigational accuracy of the system. Very broadband codes can beused to drive 1, 2, or 3 of the solenoids in this configuration. The lowfrequency content of the codes can be used for navigation and the highfrequency content for inducing current in concealed conductors. Suchbroadband codes can be designed to be approximately orthogonal to eachother in signal space and at the same time to carry significant amountsof useful information such as the orientation and coordinates for thesolenoid(s). Said coordinates may be measured by GPS, WAAS or DGPS. Saidcoordinates may be entered manually from information on corner markersor other survey means for coordination with the GCDM or GeographicCoordinate Data Base Project maintained by the Bureau of LandManagement. Further utility of solenoid navigation devices may be gainedby incorporation of radio transceivers into the design. Radio control ofthe solenoid amplitudes, frequencies, codes, and general operationalstatus would allow substantial savings in battery consumption whileimproving performance by controlling “clutter” of the spectrum within amarkout area.

For very broadband operation it is advantageous to use two coils on eachaxis with substantial leakage inductance isolating the pairs of coils.For example, the navigation coil may be a large diameter air coil andthe utility current induction coil a small diameter ferrite core coil.The axes of these two coils may be parallel but substantially displacedto minimize coupling.

Because Rayleigh fading and terrain bias will effect each Doppler sensortype differently at different times, incorporating two or more Dopplersensors, such as acoustic plus lidar, in a given direction will improvethe robustness of the system. Incorporation of accelerometers, MEMSgyros, and position fixing devices into the Kalman filter loop willfurther improve the stability and accuracy of the field mapping.Accelerometers may be used to gate out physically unreasonable Dopplerestimated velocity changes. Such changes may be expected because themapping locator will often be used outdoors and flying insects will passthrough the Doppler beams and the velocity of the insect will bemeasured for a short time. If there is no measured acceleration changeon the rising and falling edges of a velocity spike, it is reasonable tointerpret the spike as interference and interpolate the velocitiesbefore and after the spike across the duration of the spike. An improvedmapping locator is disclosed, utilizing and array of flux antennasemploying multi-point field sampling in said locator to determine thelocation of objects of interest. Advances in numerical computing power(Moore's Law) allow the cost effective use of larger numbers of antennacoils compared to prior art cable locators.

The protection afforded our invention should only be limited inaccordance with the following claims.

1. A portable locator, comprising: a housing; an elongate supportconnected to the housing, the elongate support having an upper segmentand a lower segment; at least one sensor node, the sensor node beingconnected to the elongate support and including at least threesubstantially mutually orthogonal coils; a structure that allows thesensor node to pivot relative to the housing including a hinge assemblyjoining the upper and lower segments of the elongate support; andprocessing circuitry at least partially mounted in the housing capableof receiving signals from the coils induced by electromagnetic emissionsfrom a buried object, determining a location of the buried object, andproviding an indication of the location.
 2. The portable locator ofclaim 1 wherein the one sensor node is connected to the lower segmentand the portable locator further comprises a second sensor nodeconnected to the upper segment, the second sensor node including atleast three substantially mutually orthogonal coils.
 3. The portablelocator of claim 1 wherein the three coils share a common center and anaxis of the elongate support extends through the three coils.
 4. Theportable locator of claim 3 wherein the three coils are surrounded by aspherical shell.
 5. The portable locator of claim 1 wherein each coilsurrounds a corresponding ferrite core.
 6. The portable locator of claim1 wherein a pivot axis of the sensor node is substantially perpendicularto a longitudinal axis of the housing.
 7. The portable locator of claim1 wherein the three coils do not surround any ferrite cores.
 8. Theportable locator of claim 1 wherein the hinge assembly includes a lockmechanism.
 9. The portable locator of claim 1 wherein the hinge assemblyconnects the elongate support to the housing.
 10. A portable locator,comprising: a housing; an elongate support having an upper segment and alower segment, an upper end of the upper segment being connected to thehousing; first and second sensor nodes each including at least threesubstantially mutually orthogonal coils, the first sensor node beingconnected to the lower segment of the elongate support and the secondsensor node being connected to the upper segment of the elongatesupport; a hinge assembly connecting the upper segment of the elongatesupport to the lower segment of the elongate support that allows thefirst sensor node to pivot relative to the second sensor node; andprocessing circuitry at least partially mounted in the housing capableof receiving signals from the coils induced by electromagnetic emissionsfrom a buried object, determining a location of the buried object, andproviding an indication of the location.